I guess I was asking how do these people, make the torrent say their username?

You just name the torrent with your username when you make it. They mostly follow an agreed common format to include all the relevant information in the torrent name like: Name.Of.Movie.Year.Resolution.Source.Audio.HDR.Codec-Group

And how do peers know it comes from them?

This is usually by trusting the tracker that indexes the torrent, which is usually only possible with private trackers.

PTs have scrict requirements for publishing torrents on their tracker and will tie torrents to a user on their website so that it’s impossible for other users to impersonate a group/user release name.

On public trackers, people can just lie since there’s no limits on publishing torrents and its up to the mod team to handle fake/bad torrents. Piratebay used to actually be pretty good at this with verified usernames but now I’m not so sure.

The easiest way is to join a PT which will report the same torrent hash for the same torrent on a public tracker. If it’s identical to the PT torrent, that means its from the genuine group/user.