Okay, look, I understand that people are resistant to watching any Youtube videos as citations on the board. Much less hour long ones. I know, I understand, I get it, but the most comprehensive source I know of for explaining antifa is the video The Philosophy of Antifa. Possibly the only other source that comes close is the short book Antifa: The Antifascist Handbook, and honestly I think the video does a more thorough job of citing influential sources (including said handbook) than the handbook itself, and is definitely better at explaining it to laypeople. So your choice between ordering and reading a 200 page book or watching a one hour video I guess.
The very, very short version is that there is no “Antifa” organization. In fact, when Trump said he was going to “police antifa as a terrorist organization” it immediately became a popular joke on the left to say you’re the “CEO of Antifa” because Antifa has no permanent, widespread, consistent organizational structure. It’s a loose collection of independent groups of people that respond to certain fascist actions. It’s also not strictly true to say it’s anarchist, or communist, or any other ideology. Even (neo)liberals can be Antifa, but due to their personal politics often won’t participate in it. However, in practice in the US the people doing things under the Antifa banner are likely to be Left-Anarchists, with some other far-left people interspersed such as Marxist-Leninists and such.
Antifa is based on the phrase “Antifascist Action” so (while nobody would naturally say this) it’s in some ways more accurate to say you “do an Antifa” than “are Antifa”. Antifascist action is a philosophy and set of actions you can perform, some of them even without organizing with others, and this is a wide range of things: it can be counterprotesting, or joining BLM protests, but it can also be less visible action such as (and all of these have been done withing the last 2-3 years alone):
Infiltrating far-right groups to gather intel and create dossiers on their movements
Tearing down far-right propaganda posters
Influencing legislators to not support certain legislation
Creating resources or directly providing counseling or support to facilitate the intervention, rehabilitation, and peaceful deconversion of fascists (especially ones that they may be related to or close to in other ways)
Flagging their webpages to get advertising pulled/their hosting revoked/their funding removed etc
Getting fascists fired from their jobs by calling their employer’s attention to their publicly hateful behavior
among other things.
Now, all that said, obviously there are people more into doing antifascist action than others, and these people organize, and often there are regulars who meet up and such. So in some sense there are multiple organizations with Antifascist principles with semi-regular attendees to their events, so there is some sense in which there are multiple Antifa organizations, but it’s not an organization in the same way as some formal 501(3)(c) or even something like the “Milwaukee Men’s Choir” or whatever is. It’s more an organization in the way you and your friends getting together to do a book club every week is an “organization.”