Well tbh I havent bought 2k21 yet because they will nerf or buff certain builds.
Youtube is the best reference for builds though. It really depends on your play style but if you want to play with randoms then a big man that can rebound and shoot long range will attract people to play you. Or a 2 way slasher that can attack the rim but those are a dime a dozen.
I recommend a 6'10" power forward and dump a lot into defensive rebounds and mid range/3 pointers. If you have good rebound stats and good long range stats it attracts randoms to join you.
In quick play I like playing big slow centers like jaxson hayes, not really into shooting at all, I just try to do lots of defense and get to the rim