Does anyone know how to build a bot filter like Brave’s, using JavaScript?

I was curious about how to create one like Brave does. It's different from those used by reCAPTCHA or Cloudflare and works very well on mobile.

Here's a screenshot I got from the Brave community.

Ref: https://community.brave.com/t/confirm-youre-not-a-robot-pops-up-for-every-search-result/363519