You are being downvoted because the current understanding here, AFAIU, is that there is no good identifier of the "they" where you say "they purport" because there does not currently exist an organized group that identifies themselves as "ANTIFA".
If you are aware of sources that indicate otherwise, would you mind sharing them?
There are definitely organized groups that identify themselves as "antifa" (e.g. [1]). I think you're getting at though is that antifa is not a singular organization but rather a collection of autonomous activist groups and individuals who share a roughly similar set of ideology and tactics.
Not always, but sometimes. They don’t seem to have a designated modus operandi.