Tip: Change the Preset dropdown. Also try each with 180 rotation.
The algorithm is as follows:
It would appear that predictable patterns come from random numbers - or "chaos". In truth, some areas cannot be reached regardless of the points chosen and thus certain patterns are guaranteed. Some patterns rely on certain areas having a lower probability and hence you get different levels of "shading".
Chaos was written in Zig (because it's very fast and I like Zig) using minimal-zig-wasm-canvas as a starting point. Chaos source on GitHub.