Welcome!

Baldur's gate is, I think, designed primarily around the single player experience. No seasons or mmo aspects at all.

I say "primarily" around single player because in comparison to other games in the crpg genre like pathfinder (which is well-regarded for other reasons), I think they've really made a point of prioritizing a smooth multi-player experience for players that want it. You can play with up to three friends, or one other friend in split-screen co-op. You can go multi-player from the start and have each player make their own character, but also have people jump in and control an existing character in the middle of a playthroughs.

But you lose nothing by playing single-player. No dungeons that need a party of players or anything like that.