"antifa" isn't paramilitary or any other sort of group. It's a contraction of "anti-fascist" and is simply a tenet that many people hold.

"Proud Boys" are an organized domestic terrorist group, not a label for a set of beliefs.

There was real, violent, semi-organized left-wing radicalism in the USA in the 1970's. Not since. It may return! But it hasn't yet.

You might want to be more skeptical of the media you're consuming that led you to believe this. Or the persons you've been talking to.