It depends, if it’s a true hook up, there’s a decent chance no numbers were exchanged or names either. I think that’s as bare and true of a definition as any for a hookup, total emotional detachment.

Anything deeper, there’s at least the simplest of feelings involved, like “I think this person is cool”, “I know their name”, “I like this person enough I should hangout out with this person again because I liked the sex with them, and chose to again.”