Ok I think people are a bit confused here

So, you got your Android System Update, this is provided by your phone maker, like Xiaomi for example

they have their own version number, and usually this brings Security Patches and also new features/android version

Then, you have Google Play System Update
Not to be confused with Play Store App

This is part of Google Project Mainline

Google is tired of OEMs being slow to push updates, so a part of it is controlled by them and they can push the updates

This can bring new features/fixes
(but also break apps, latest Jan update breaks some modded apps)

Lastly, you have your play store app and it's own updates

ApkMirror is fine too, those 2 are the ones I use personally

my default go to is ApkMirror, it's run by the news site "AndroidPolice", they're safe

APKPure is good too