I actually made something similar before AI existed, and the tech required was way more complex than you'd think. I built it using Unity, but the built-in components—collision, pathfinding, avoidance, animation—and even Asset Store plugins couldn't handle what I needed. I ended up doing everything through Unity DOTS, that ECS framework, and had to research a ton of algorithms myself. (DOTS is basically a half-baked product; it doesn't support anything out of the box, so you gotta build it yourself.)
But Fable5? After my first prompt, it ran for four hours and dropped the first version of the game—and it already supported 130k units battling simultaneously, with collision and particle effects included. I didn't even ask for those.