This website contains age-restricted materials including nudity and explicit depictions of sexual activity.
By entering, you affirm that you are at least 18 years of age or the age of majority in the jurisdiction you are accessing the website from and you consent to viewing sexually explicit content.
It uses other signals too, like what other sites you’ve visited with that checkbox on it, what CloudFlare has seen your IP address doing in the past, etc.
The google one is able to see if you’re logged into a google account and take that into account.
There’s even a new variant of the Google captcha that is invisible and doesn’t even bother to show a checkbox.