What do they follow?

Guessing you might have meant IP address, not MAC address. Without a VPN or w/e, the IP for most ppl is assigned by the ISP and damn near pinpoints you. Not exactly, but it's real close.

MAC address on the other hand, that you can change yourself (prob).

With a legit VPN, they don't get your IP. They do get other fingerprinting signals tho! Maybe lots of them. If you ALSO mask those, that makes your abuse likelihood score even higher. IDK waht the threshold is, but it seems anymore that a VPN alone is "almost" a surefire ban. Maybe some ppl get it to work, but I think that's b/c reddit can fingerprint them in enough other ways.

There's a whole ass industry about this! It's called identity resolution. Most of the big site use it now. It uses any and every signal to ID you. Even things that are near impossible to hide. They're shitweasels.