I think it does make a difference: there are tons of Android ROMs (or even Linux distros like PostmarketOS) that run on many different Android smartphones. But I don't see the equivalent for iPhones.

Isn't that because iPhones are more closed somehow? Maybe there is another reason, I don't know to be honest.

That’s down to different reasons:

1. You can still reuse the binary blobs that make up the drivers.

2. Android has unlockable bootloaders whereas iPhones do not.

However the binary blobs are not always legally redistributable or even reusable.

They don’t also help if you need to support any version of the kernel other than what shipped in case there’s any ABI differences

Thanks for the insights!