How work on recaptcha v2

I know 2captcha service works with human. They answer captcha with site member, But my main question is how they get recaptcha? On their api documentation to pass recaptcha to them, Requirements are: Url of a site that need to answer captcha. Google site-key.

After that they will send validated token that you can submit into the url that need to solve captcha.

Im trying to make a php script where i can solve recaptchav2 from other url. Thank u.