I learned an enormous amount from in-person game jams by chatting with other game developers at the event, most of whom were more skilled than I was. I've also found game jams made it easier find collaborators, easier to get feedback from other designers, and easier to see how other people approach the same theme.

> But if you don't know how to make a game, where does the knowledge come from?

If you're at the stage where you feel like you don't know how to make a game, you might find value in doing the exercises from the book "Challenges for Game Designers" by Brathwaite & Schreiber.