How is the situation with app updates in Android 7+?

Question

Android

I use Windows 10 Mobile and have never used any Android device in my life. But the cards are such that Microsoft cares about Android more than Google itself, so I'm switching to it.
The problem is this: for example, I want to install a modified APK from a well-known site. Will Play Store update this app to the original version from the Google Store? Or not?
In that case, is it not possible to side-load modified APKs on Android OS (not including development mode)?

Answer 1

No. Will not update.
Why is it impossible to load the site?

Answer 2

There are two options: if you want automatic updates - install software from the playmarket; if you install from one well-known site, then the updates will be in manual mode (deleted the old version, downloaded the new one, installed it, configured it)