Inherited a service with zero documentation and the person who wrote it left the company back in 2023

Three weeks into my first job and this landed on my plate because "you're the new backend hire, this is yours now." No README, no comments explaining the weirder decisions, one Slack thread from two years ago that references a bug nobody ever closed out. I've been reverse-engineering behavior by reading logs and adding print statements like it's a crime scene. Nobody around me has full context either, so it's not like I can just ask. If you've been handed something like this, how did you even decide where to start reading first?