So, how does adventure mode work?

[Baldurs_Gate_2]

Do you get all the level ups / items right to the campaign or do you have to fully do the area you have chosen?

[Suzaku]

Adventure mode works like a slice of the campaign, having you access a single biome per run. Everything you had except for quest items carries over, and everything you earn except quest items is retained as well.

[Trichouette]

Campaign = world 1 -> labyrinth -> world 2 & world 3 -> last world (spoiler)
Adventure mode = world 1 or 2 or 3, you choose

keep in mind each world has 2 variant
You play your character, and can get more loot