BetMGM Poker is one of two US “skins” running on the partypoker US network engine — the other being Borgata Poker. Same tables, same tournaments, same underlying liquidity; the only real differences are branding and which loyalty program you're plugged into.
It's licensed and regulated in New Jersey, Pennsylvania, and Michigan, with those three states sharing one merged player pool. It's also licensed in West Virginia, though that market is smaller and isn't confirmed to share the same combined liquidity as the NJ/PA/MI pool.
As part of MGM Resorts' broader BetMGM brand, every hand you play also earns rewards in the wider BetMGM/MGM ecosystem — not just poker-specific perks.