It really depends on a bunch of factors: the environment, company structure, your role — you get the gist.

Early on, most time goes into writing new code, with some design docs if the team is structured — this is as simple as writing a README.md or having a static documentation website.

Once you have users, writing new features slows down, and more time shifts to bug fixes, refactoring, and keeping things running. In startups, this transition happens later since growth takes priority. I haven't worked for any large companies yet but I assume they tend to balance maintenance earlier.