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.
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.