I assume you're asking how crpgs play as a whole, since BG is just that.
Its not a MMO, its essentially single-player but can be played coop with friends or randoms online. Aside from that, plays like a normal RPG, but mechanics and storyline in this genre tend take the frontseat, being much more in-depth than 'mainstream' RPGs like Skyrim or pseudo-RPGs like The Witcher 3 (though the level of that depth obviously depends on the title).
Using BG as reference, in such games you're often free to permanently kill 99% of NPCs you see, you have tons of dialogue choices, all of which affect the progression of the game, proper character building is crucial and you cannot afford to just put points wherever, as combat veers towards a more tactical nature. One key difference between older BG games and BG3 is that older were real-time combat with pause, while BG3 is turn-based.
One reason people are excited to BG3 is because it promises to have the depth of traditional crpgs coupled with the immersiveness and production value of mainstream AAA games, something other crpgs are often lacking as those aren't their main focus.
