How banner ads work?


Hi! Im begining my way as an app builder.

I saw several apps and i noted that some of them put a banner and it reminds the same ad stays a while and then changes to another ; but others put the banner en when you click on something in the app the ad change.

So; i spend the same time in two diferrent apps.
The first one shows ad1; then like a minute or two after showed ad2 and then showed againd ad1.

This count as 1, 2 or 3 impressions?

Then i spend the same time in app 2 and it does a screen change ( i think). So, in screen 1 there is one ad and screen 2 there is another.

In screen 1 the ad remains the same and in screen 2 the ad changed everytime so; the question is; every time i go from screen 1 to screen 2 and reverse its generates ad impression?

Then i also saw that ad from screen 1 is from google and. From screen 2 its a difefent network.
This could only be with two networks? Or you can do the same with admob? ( put 2 screens and two admob banners)