Someone explain Vader being Luke’s father to me

I’ve never watched the Star Wars movies, but the one thing I thought I knew was:

Luke finds out Darth Vader is his father right before Vader dies.

This weekend I watched the last 30 minutes of Return of the Jedi and learned a few things.

This reveal doesn’t happen right before Vader dies.

I watched Vader die, it was pathetic. But apparently he tells Luke he’s his father in one of the earlier movies.

Luke already KNEW Vader was his father before Vader told him???

“Sam…….*kkkhhhhhh* …. This blog sucks…….*kkkhhhhhh*”

I need this to be confirmed, because I can’t even find it online, everything related to ‘Luke / Vader father’ is about how misquoted it is, Vader actually says “No, I am your father“.

This weekend someone was trying to tell me that Luke always knew Vader was his biological father, but since he turned to the dark side, Luke didn’t consider it to be true anymore? Something like that. Then Vader’s big “I am your father” was just a metaphor for like… “Hey, I’m not that bad” or something like that. He explained it to me five times and I still didn’t get it.

That can’t be true. Someone tell me that’s not true. I’ve never had any interest in catching up and watching all of the Star Wars, but if this is true, I’ll have less interest.