16 random bytes is not a valid UUIDv4. I don’t think it needs to be in the standard library, but implementing the spec yourself is also not the right choice for 99% of cases.
If you generate random bytes, which are unlikely to conform to the UUIDv4 spec, my guess is that most libraries will silently accept the id. That is, generating random bytes, will probably work just work.
I didn't say about 16 random bytes. But you're almost there. You generate 16 random bytes and perform few bitwise operations to set version and variant bits correctly.
Not that it matters. I don't even think that there's a single piece of software in the world which would actually care about these bits rather than treating the whole byte array as opaque thing.
