What's the actual gameplay loop?

I.e. what's the goal, how do you know you're doing well (or not), what makes it fun etc?

The loop is all about adapting, experimenting, and seeing which combinations work and which don't. Right now it's designed around mini-games which can have a different goal per game -- as a quick example I'm currently building agent tic-tac-toe but hidden trapdoors and power-ups

So strange. Like, it doesn't sound like there really is a proper loop that you have figured out. But this actually sounds exciting - like when you read about how certain game genres were emerging and game designers were almost accidentally discovering what is fun.

I don't think the Sims had that

The Sims had multiple goals for the player.

You had basic needs to fulfill, career advancement, relationships, and family generations.

Each of those fulfills the game loop.

In these sandbox games you just make up your own little stories and have fun watching them play out.