I'm not an expert here so please take what I say with a grain of salt.

It's my understanding that what's included in open source Android (AOSP) is FAR from a complete product and there is quite a bit of Google closed source/proprietary software that goes into the mix before it's shipped as Android (think Google Services.)

So, while you could fork AOSP and try to use that as a basis for and alternative mobile OS, it would require quite a bit of work on top of the AOSP code. This is what's done by custom ROMs like GrapheneOS (ironically Pixel devices only) or LineageOS for example.