RimWorld was never really a Unity game. Most of the game logic was always running on their own framework called Verse (not related to the Epic Games language, it's a Firefly reference).

They basically made a homebrew ECS hybrid to support all the complex game logic and AI behaviors. It takes care of NPCs, items, health status, combat, random event, basically the whole game. The only things Unity handles are low level systems like input handling or rendering that are hard to get right and good enough as they are.