Activity vs Fragments again. Separate logic and make app smooth

sorry for asking this question again. I have read all related articles, but haven't figured out what is better to use and when.
My app is going to be online store native client.
So I have a bunch of different screens in my app.

1. First Screen. It will be main screen like in the most online stores. There will be image slider, last new products, sales, ad and other common stuff for store.
2. Second screen. There will be simple list of categories in the store, without any ad,sliders and so on. Only list (RecyclerView).
3. Third screen. Will represent description of product, price, photos and other.
4. Fourth screen. Login and sign up screen to let user login in it's account.

There would be a lot of other screens required for native store client.
I have some questions about this.

1. I have watched the video, where lecturer said that in their app they are using only one activity for all app except settings and payment. So it makes app smoother, responsive because fragments are more lightweight that fragments. Okey, in this case we have base Activity, it should implement all callbacks from fragments to handle data and replace screen with another fragment.
   We can use event bus to make it easier to with callbacks, but there is another problem.

2. How my activity layout should be built, it should have one fragment container and we can replace whole screen with another fragment or activity should include several fragments. For example for each image slider, ad, last products use its own fragment?

3. If we will use one container, so in this case we change whole screen. Is there any benefit of using fragment in this case ?

4. If use several fragment per screen. We have to put them in parent framgent ? In this case we are dealing with nested fragments. In most cases it is bad practice to do so.

5. Or we have just to know about all fragments currently present on the screen and when we have to change all screen view remove all old fragments and add new ?

6. What about nested fragments? Is bad to use them, and I have to do my best to get rid of them ?

Please explain how to build app to make it responsive but also to make code pattern and best practices oriented.
Thanks everyone for the help in advance.