[Phylum]

You don't need formal training to play music. Being a high level instrumentalist in an orchestra or something* definitely requires some kind of intense formal training over several years, in 99% of cases at least.

More modern music styles requires less instrumental finesse, and can definitely be largely picked up by ear. I discovered yesterday that I can kind of comp piano chords, despite never spending much time with a piano. I think it was part having a good ear, part knowing the song and part being to do a 5-second musical analysis before we started. I knew where the cadences were and what the function of each chord was, so I could base what I was doing really strongly around that in terms of voiceings and auxiliary figures. Also I didn't have to actually think about what notes were in the chords.

What I'm trying to say is, learn music theory. It will make you 100x more versatile as a musician. Technical analysis in tandem with a trained (or in my case natural) ear is a powerful thing.

*e: also includes lots of crazy jazz musicians.