Nobody talks about SEI anymore but their Capability Maturity Model informed and/or aligned with how I approach new-to-me projects whether that 'new role' is internal or a new employer.

CMM says that an engineering team whose processes aren't written down is level 0, and writing them down, even if they are batshit, gets you to level 1. Which leads to a fun bit of catharsis with the older employees where they get to say things like, "and then a miracle occurs".

Also writing things down gives someone else a peek into what's going on in your head and they can correct bad assumptions you have before they get cemented and take more effort to dig out of your thought processes.

So I always start with fixing the documentation, the runbooks, the CI process. It's a deliverable you can engage in without breaking production, it demonstrates mastery, it fixes a pain point that the more professionally mature members of the team care about, which gets you brownie points with the right sort of people. And it makes it easier to onboard the next person, or pick back up a project that has been on the back burner for several quarters.

TODOs emerge from the documentation or get explained away as unnecessary or wontfix. By the time you're touching something you have a better idea of why things are like they are, so you make up for 'lost' time.