I have 16 Core Beta. I installed the Beta version, because I liked how 16 looked, but that was like 2 months ago, so 16 stable was not a thing yet. Do I need to reinstall to get the newest stuff or do I get it just updating my system? I guess apt will receive all updates, however, I suspect there are always some configurations that don't really relate to apt upgrades that I wouldn't receive without reinstalling (GUI stuff for example).
Yes, clean flash is recommended by devs
It's recommendable, but not a necessarily a must. The Beta will get updated to stable and you get the upgrades like everyone else does.