Ask HN: How can I learn from an existing codebase?

I have the opportunity to learn backend by studying a codebase of an abandoned project. It's a company project, and I asked the devs who made it to make sure that I'm not doing something they don't want me to do.

Problem is, I barely qualify as a backend developer. I'm a junior who was hired as a fullstack and yet haven't touched backend for as long as I'm in this company. The only things I made was following tutorial videos and documentation, which are obviously comparably simple.

Which means I'm excited that I can finally sink my teeth into a real, breathing project to try to extract as much knowledge as I can. From how to structure it, to how they handle middleware, the requirements they had to fulfill, etc.

How can I start growing my skill by doing this, though? I can either a) treat it like a video tutorial and just type the same things they wrote and see how they click, b) try to get the gist on where things go or ask one of the devs who authored it to show me around, but that's company time and I'm sure he has his own workload to do and the last thing I want to be is a burden.

Do you have any recommended way to study and grow your personal skills by reading other people's projects?