PITA transferring all my whatsapp messages from iOS to android though. Everything else was pretty seamless.
I guess on iOS it wont let you back them up to Google Drive? Now you're on a good OS (
) you don't need to worry. Have it set to backup each night, when you change phones to another Android you can just press restore.
Noob alert, on ios I could double tap the home button to be able to close apps down in the background. How the sodding hell do I do this on android as I can't remember. Lol
Generally you don't. Can I ask why you did on iOS? I assume to save battery? Android manages it all for you now and it can actually cause more drain if you kill them all the time. A lot of them will need to be open, or you open them and it takes more to open them from fully closed. If you must, press the app switching button (by default the left one) at the bottom it says 'Close all'. Hit that. As I say, I would try and get out of the habbit tbh.
The other route, go to settings, Device Maintenance -> Battery and hit 'SAVE POWER'. If you know something need to run in the background, you can set it in 'Unmonitored apps' on that same page.