Yeah, generally they're playing another D&D-style game, or more rarely a game that's an entirely different genre. Game like Pathfinder (2e is the big internet favorite atm) and OSR games are designed to be basically the same niche genre of "dungeon-crawling, combat-centric high fantasy" but with different rules. Other games like Dungeon World use the same setting concept but the rules are more narrative-focused with less emphasis on hours-long combats on a grid. Further afield are generic systems like FATE, GURPS, Savage Worlds, etc which are flexible and can be used to play whatever genre you want, so they can be D&D-fantasy or cyberpunk, superheroes, historical fantasy, "action movies", etc (though most of these tend to be best at one or a few things). Or you can just play something totally different like Lancer, a game set in the space communist future where you fight slavers and fascist remnants in giant mechs, or Thirsty Sword Lesbians, which I know very little about except that the name is funny and that it is very queer.

Personally I use GURPS, because it does a lot of things I like and because it can be used to run different genres with minimal effort. Things like extreme customizability, crunchy combat that actually has multiple options worth choosing that aren't just 'spam attacks', no HP bloat, point-buy characters with no classes (unless you want them), I could go on.