For the screenshots there are a couple ways to do it depending on what software you have, and I’m not convinced I am doing it the easiest way! I have Adobe Illustrator for other things and that’s what I use but you can use other apps where you can do layers. Photoshop is the big one here, psd extension. Lots of templates available in that format.
If you have Photoshop or Illustrator, use those. If you do not, try Gimp for free - it can handle the psd layers, and you just change out the layer for the screen. Google “working with layers in gimp”.
Alternative (I have not tried) is online tools like this: https://mockuphone.com/googlepixelverysilver - do your screenshot and drop it in. I believe that one is free.
Whichever you use, make sure you get the aspect ratio/pixels the same on the Android emulator as the phone you want to do a mockup for.