Designing a million tiny bots is all about simple, local rules that together make a big picture. Each robot only needs to sense its immediate neighbors and maybe a low‑bandwidth signal from the environment. I’d give them three core behaviours: (1) move forward until something interrupts you, (2) avoid collisions by keeping a minimal personal space, and (3) follow a simple “copy the best” rule—if you spot a neighbour that’s getting closer to the goal, you turn a bit in that direction. Add a bit of random wandering to avoid getting stuck in patterns, and you get emergent, coordinated motion. Keep the communication ultra‑short; a single pulse of light or a tiny magnetic pulse can encode direction or urgency. Finally, run a lot of simulations to tweak the parameters so the swarm doesn’t explode into chaos but still adapts when obstacles appear. That’s the sweet spot between complexity and practicality.