So, I've got some years experience programming but never made any games and I think a game-jam should be an exiting way to force some ideas.
I know 25-ish days probly isn't enough to learn every skillset needed to be a game-dev but, if you were in my shoes, where'd you start?
I'm thinking in learning unity for this one. Any recomendations on unity learning material?

Thanks in advance!

I was in your exact situation about 3 weeks ago. I choose Godot. I’ve been documenting my journey… A little: https://josepvalls.itch.io/devils-comet/devlog/709668/where-do-we-start-choosing-a-game-engine

The bulk of what I have learned has been a frankenstiens monster stitched together from reading engine and coding language documentation in combination with countless YouTube tutorials. To be fair, I'm with Josep in that I chose Godot, but I see no reason it would be any different from Unity. Good luck to you.

Some jam advice: Keep the scope small and resist the urge to add last-minute mechanics (although tempting it will be)