Everyone gets Movement, Action, Bonus Action, Reaction (In Game).
Reactions (default) are basically taking attacks of opportunity, can also be spells like Hellish Rebuke. Toggles on the far right hotbar.
Dash doubles your move, everyone can do this as an Action (instead of casting / attacking). The Rog 2 ability OP asked about, lets you Dash as a Bonus action, allowing you to Cast / Attack as well.
Yes, indeed. The thing is, in the tabletop version, you can Move + Dash (Action) + Cunning Action move (Bonus Action). I do not think you can in BG3, which is not a big deal. Like you said, the Rogue still has a good bonus being able to attack with an extra move if needed.
EDIT: in the current build, Rogues seem to have two Bonus Actions, which does allow them to do a lot of things ! Including movement combos.