The furniture aren't very damaged, just a bit worn.
I don't have any knowledge or experience on the subject, I'm just trying to revive them a bit. I'd rather not unscrew it or take it apart.
I found information on the topic, but I'm not sure which are relevant to my case, and I don't want to mess it up.
I think I have to use wood finish, but I don't know of which type whatsoever. And I'm not sure either wether I have to scrub it.
Can't tell what sort of varnish, if any, is on there, but it looks like it's come off in the worn places you care about anyway. Without a complete refinish job, you can try touching up much of the damage you picture with a swab that's been dipped in a dark oak (or red oak) stain. Just put a little stain in the places the finish is gone, wait 20 min. and wipe off gently. The result will not be as good as a complete strip, sand, restain and refinish job, but with the correct choice of stain color I've seen this method take things from fug-ugly to really not too shabby. If you go this route, be sure to test it on a small, inconspicuous area first.
