IWTL how to train my brain as a "muscle"

So I know maybe it's a pretty dumb question and probably been asked before but I was wondering what are the most important points, how to PROGRESS and how does it work. Sure I already know that reading probably trains it, but does for example playing an instrument (I play the guitar) or trying to be ambidextrous ( I heard that it can literally form neurons and have an impact on your overall brain power), doing complex puzzles or math, playing chess or whatever. I heard this expression a thousand times that it's a muscle but how do you train it then