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Which Layout to use with Fragments?

Recently delve into fragments and from what I understand to create a fragment you need a java class and the fragments layout. This makes sense. However what I cant seem to wrap my head around is what "container", or layout do I use to store/insert the fragment? In android studio you can use this to insert fragments, or you can use any other of the layouts. But which one is ideal to use?
Also I saw in a reddit post that I shouldn't be using fragments at all and that its preferred to instead use Frame layouts and play around with your views visibility for the desired effects. Is this true?

You're slightly over-complicating the concept of Fragments.
Fragments, like Activities, don't actually need a dedicated fragments layout xml file. If you choose to do so, you can create the entire layout through Java code, not that I will understand why you'll choose to do so.
So for Fragments, you don't need a Java class and a fragment's layout file. The only requirement is the Java class, and the layouts file is just the preferred approach to inflating a layout, similar to how it is for Activities.
As for your question about the container of the fragments, it's really a matter of your app's design.
You can add a Fragment to your Activity or other Fragments, through the FragmentManager in code or through the <fragment> tag in your layout.xml files.
Neither of those are the best way or the preferred way, since it really depends on what your app needs.
Using the <fragment> tag will cause that fragment to always be added whenever the layout is inflated. This is actually VERY bad if your Activity requires dynamically switching Fragments due to your use of things like ViewPagers, Tabs, Drawer Navigation, or etc. However, it's GREAT if there's no need to dynamically switch fragments and for that specific Activity or parent Fragment, this fragment is a fragment that's always loaded.
For example, let's say you designed a flexible AddNew Fragment that's used in a Dialog and an AddNewActivity. Due to reusing the same screen and code, you decide to make this part of your code a fragment so you can insert it inside a DialogFragment or into another Activity. But, for those DialogFragments and Activities, the only Fragment it'll have is the AddNewFragment, so it'll make sense to just insert that fragment into the Dialog layout and Activity layout through the <fragment> tag.
As for the option with Java code, the preferred approach is to use a FrameLayout. But there's no need to play around with any View visibilities!
The common approach is to just use:
<FrameLayout
android:id="#+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent" />
A FrameLayout is used because it's going to be the container of the Fragment. In other words, the Fragment will be stored inside of this layout.
So in Java code, you can simply use this code to replace the Fragment inside the container with your new one:
getSupportFragmentManager().beginTransaction().replace(R.id.container, AddNewFragment.newInstance()).commit();
Optionally, you can use add() instead of replace() if you want the fragment to be placed ontop of another fragment within the FrameLayout container.
So yes, to give a decisive answer to your question, there's no ideal way to add a Fragment to an Activity or another Fragment. Each option has it's benefits and drawbacks, with some working better for certain situations and others working better for others.
In the end, it really depends on what your App needs. If you need your Fragments to be flexible, so you can switch Fragments, then this must be done through Java code, because fragments added through the <fragment> tag can't be removed at runtime. However, if you don't need your Fragment to be replaced and it's definitely always going to be showing the same Fragment, then using the <fragment> tag removes the need to write extra Java code to load the dedicated Fragment.
One thing I really do need to point out is... that reddit page you read about is wrong. The 'preferred' way to use Fragments is not to use FrameLayouts and play around with View visibilities. I actually have no idea why there's even a need to change View visibilities.

You can use the new androidx.fragment.app.FragmentContainerView to get a better performance than FrameLayout.
Here more information.

You could use the Fragment layout, but this isn't very flexible. If you're just showing one Fragment, it should work, but using a FrameLayout and inserting your Fragment into it works better as this allows you to change them on the fly.
You may see a FrameLayout with the id R.id.container or something similar, and what this is used for is the Transaction.
For example, if you want to insert FragmentOne into your layout, you can just do this and it'll put it into R.id.container.
Fragment fragment = new FragmentOne();
FragmentManager manager = getSupportFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.replace(R.id.container, fragment);
transaction.commit();

How can I add multiple activity to the same screen in android

I want to divide my screen into four parts and add activities in each part. I am not interested in using fragment. Each activity should behave independent of other. Attached photo is showing what I exactly want to do.
In each child activity I want to add VideoView or WebView depending on the selection from menu item.
How can I do it. I didn't find any way to add activity to an activity.
Thanks :)
PS: Activity means Activity not fragment.

i strongly recommend 'using fragments' in your case as ActivityGroup is deprecated in API 13..

The only way you can do it by using fragments. Here is the simple example on how to add multiple fragments on single activity Link
Edit link

You can achieve this design by using four fragments, one for each child view inside a single activity.
The recommended way is to create a single XML layout for your activity, and create four fragments inside that layout.

Using Toolbar in Fragment or in Activity

I am using Android-ObservableScrollView library. Everything works great, but I have activity for holding fragments, so all views are encapsulated in the fragment. In activity there is only FrameLayout for holding fragments.
So I need to use Toolbar in my application, I have several ideas how to implement this.
Use Toolbar in activity, in this case my layout will have FrameLayout and Toolbar. In this way I have communicate with activity whenever I need to do something with toolbar, I can also obtain it by using getSupportedActionBar() from fragment.
Use Toolbar inside fragment (in its layout) setting in each fragment view creation. And each time I change fragment I have to add new Toolbar to the activity. In some fragment I am going to have different toolbars but not in all. Is it good approach to store Toolbar inside fragment.
The problem that I can see in using second approach, if there will be more than one fragment on the screen there will be also several toolbars.
Please suggest what will be the right way in this case.
Thank you.

You should use first method.
While using first method will have less problem then second one because in second method you have to define toolbar for many times which is not good programming.

Why should I not add Fragments to the activity layout XML file?

Edit: source : http://developer.android.com/training/basics/fragments/creating.html last parargraph says :
Note: When you add a fragment to an activity layout by defining the
fragment in the layout XML file, you cannot remove the fragment at
runtime. If you plan to swap your fragments in and out during user
interaction, you must add the fragment to the activity when the
activity first starts, as shown in the next lesson.
Thanks in advance.

You can use fragments in two ways ,
Static Fragments
Here you can define the fragment in whatever the layout file you need. Only thing is, that defined fragment can not be change at the run time. So, re-usability will be issue here, you cant take the advantage of the re-usability of fragments in this case.
Dynamic Fragments
Here you can define a place holder(frame layout etc) on your layout and you can add/replace whatever the fragment you expect at any time while your activity is running. This ensure the re-usability.
Also you can use backStack if back navigation is required.
So, it depend on your requirement.

If the fragments are defined in Xml you won't be able to change them at runtime; in that case I'd suggest using an <include layout="#layout/my_inner_layout"/> tag, that is not only better performance-wise as it is easier to use (you can access the views defined in my_inner_layout.xml using findViewById() method from the host activity).

Nested fragments good practice?

I wonder if it is good practice to use nested fragments. Consider, there is Activity. I need to place viewpager at the top of it and list view above viewpager.
It would be better to use two containers one for ViewPager, another for ListView, it is clear.
But the question is is good practice to use antoher fragment as container for two nested. I mean that this fragment will have to layouts for two nested fragments, it will have similiar layout file as activity if there are two fragments without parent one.
So I wonder is it good practice to use one main fragment in each activity which plays container role and store all other components inside this container fragment or it is better just use several fragments inside activity without container fragment.
I would be grateful for any help. Thanks in advance.

The "container" Fragment is not a good idea, simply because it will mean two layers of nested Fragments: the outermost "container" Fragment, inside it the Fragment which holds the ViewPager, and the ViewPager itself which holds a set of Fragments.
It would be better to not have the "container" Fragment at all. Keep the rest same.