Potentially you can do "everything", but you can't do everything well. Moreover, you will lose a lot of time to, which you could have spent to become better in one or a few of the directions.

So... practically I do everything myself. My primary vector is coding and story/writing. However, I pick free music and sounds from the internet (with basic processing). I make UI based on some free assets, but usually nothing fits my idea, so I have to rework it significantly. Art is the worst - I'm very bad at drawing, but I ain't got much of an option when it comes to art assets: either paint those myself (which I do), or commission them (which I'm not doing, because I believe that the game should at least pay for itself, and my games are free :)). Ah, yes, promotion too, ain't nobody's gonna help me here :)