I don't know too much about the rules of D&D. This is all just based on what I've noticed in Baldur's Gate 3.

I've played a lot of CRPG games but this is the first where offhand attack appears to be a separate action. I'm thinking this is the decision Larian came up with while trying to make BG3 resemble the PnP style of play, where you need to usually say how many times you attack with what weapon, what you do with your bonus actions etc..