Android is not proprietary.

https://source.android.com/

That makes it rather easier...

You linked to AOSP, not Android. AOSP doesn't have things like the Google Play Store; the shipped image is substantially different and isn't necessarily open-source just because it contains open-source components - Windows contains BSD networking code but that doesn't make Windows open-source.

Sure, but the vast majority of those mods and alternative distributions are based on the sources that are available.

It is a bit like a Linux distro running Steam. Yeah, there are proprietary bits, but the FOSS part naturally happens on the FOSS parts.

You know what the "A" in AOSP stands for, right? :-)

All I'm saying is that there's a big difference in jailbreaking an OS when most of the source code of that OS is publicly available for study and experiment. Surely that is not a controversial statement?

You don't root OSes, you root devices. I learned that the hard way when I got a Samsung Z-Fold 4 and learned that the US version isn't rootable.

All this terminology is muddied and often misused. The whole term is supposed to refer to getting access to the root user, which is definitely an OS-level thing. One thing people often don't remember also is that gaining root on your phone and unlocking the bootloader are not only different things, but entirely separate things. You can gain root in your vendor ROM without being able to unlock the bootloader (common case on Amazon tablets), and you can install LineageOS in your phone after unlocking the bootloader but you still won't have root access in the OS unless you do an extra step, setting up Magisk or similar. You also have to re-set-up Magisk after every LineageOS update, which often means once a week if you do all the OTA updates in a timely manner. Gaining root is usually the easier thing and the less useful thing. It doesn't help much with getting a nearer-to-AOSP experience. You can remove some apps but there are still limits to what can be done without just flashing a different ROM with less crap in it.

When picking a phone I like to see if it's on the LineageOS devices page (you bring up a good point that different variations of the same model aren't equal, definitely watch out for that), as that means both that the bootloader is unlockable and that someone else is already maintaining a ROM for it, and hopefully will for years to come. If I just go after the shiny new hardware, chances are the bootloader is locked and it will never be unlocked, generating e-waste.