When a cookie is stored, it has an attribute of “domain”, which creates a restriction in which websites can subsequently access that cookie. For example, if you browse to example.org and it sends a response header with Set-Cookie: test=“somevalue”, then you browse to dodgysite.com, that site can not access the test cookie for the example.com domain.

You can press F12 and look at the network tab and then cookies, to see what cookies are sent and received in the http exchanges. You can look in the local data in your browser settings to find out what cookies are stored against a given site, but you can’t easily tell which cookies are used by a website because the browser will simply include all cookies for that site in every request.