A remix is a new version of an existing song, reworked by someone other than the makers of the original recording, usually a producer or a DJ. The song stays the same, but the version is a different piece of work, and that is why it carries a credit. When we see a remix label in a title, it tells you that you are looking at a version of a song you may already know, made by a different hand.
The credit sits in brackets after the title and names whoever made that version, as in "(Name Remix)". The name inside the bracket is the remixer. It is not a second artist on the song, and that is a common misreading. For contrast, a language version re-sings the song in another language and a re-recorded label means the song was performed again from scratch. A remix usually works from the original recording itself and changes what is done with it, though the exact method varies and the label does not tell you how the work was done.
A release often gathers remixes in one place: several remixed versions, sometimes together with the original, under one title. The tracklist is where you see which versions are included, and an original appearing in that list is normal rather than an error. Some remix releases are numbered as a series, and a "Vol." number marks that release's place in the series. It counts releases in the series, not the number of remixes inside one, and it says nothing about order of popularity.
You usually find these releases on streaming services under one release title, and often as a video upload with the remix credit in the video title. An official upload comes from the artist's side or its label. A remix that a fan made and posted on its own is not the same thing, and the credit and the channel are how you tell. We should also be honest about a limit: remix credits are written differently from release to release, and a title alone does not tell you whether a remix was made with the original artist's involvement beyond being officially released.