Rather than try to unilaterally deprecate and replace CAPTCHA with a single alternative, we built a platform to test many alternatives and rotate new challenges in and out as they become more or less effective.

With Turnstile, we adapt the actual challenge outcome to the individual visitor or browser. First, we run a series of small non-interactive JavaScript challenges gathering more signals about the visitor/browser environment. Those challenges include, proof-of-work, proof-of-space, probing for web APIs, and various other challenges for detecting browser-quirks and human behavior. As a result, we can fine-tune the difficulty of the challenge to the specific request and avoid ever showing a visual puzzle to a user.

Turnstile also includes machine learning models that detect common features of end visitors who were able to pass a challenge before. The computational hardness of those initial challenges may vary by visitor, but is targeted to run fast.

https://developers.cloudflare.com/turnstile/

TL;DR A bunch of heuristics that it’s hard to spoof all of. Fun side effect of this method is that if you spoof your user agent, you’ll often end up locked out in a loop. Lack of a captcha fallback is obnoxious.