Huawei, for example, has installed EMUI alongside Android. What do you call the whole? Has Huawei Android just modified it to your liking? Or how exactly is this done?
You have already answered it yourself. The manufacturers buy rights to Android to then adapt it for their devices.
So I know that under surface.
"Android" is just a base. Google brings out the OS so far.
The manufacturers can then use this and then pack their own surface over it.
However, certain obligations are linked to the use of Android, which is why the manufacturers here at least not complete freedom of madness is given.