How does the site perceive and identify users?

Question

PHP

The question is: "How does the site perceive and identify users?" How, for example, does the site on which you leave a comment know who left it? They say that you can send data from the session along with a comment to the database (image url, username, profile link, etc.). But, if, for example, a user, while being on two sites, on one of which he will change the session data (if possible), which is used on another site for commenting and everything else, tries to leave a comment with new data from the session, then it turns out that he leaves a comment on a completely different name. And if not sessions, then what? To query the database, you still need some kind of identifier.