Think of looking at onboarding to new code repo as an engineer looks at a new mechanical or electrical equipment.
You typically read the manual if any (README.md). If the manual is lost, you just gotta figure it out.
You have a vague idea of what the machine does (printer, grinder, engine, computer). So you also have a little idea of what the code is for.
Start by learing how to turn it on. In code, find the entry point.
See what each button or switch does. In code, look for the interfaces. This is where it gets difficult. Perhaps the interfaces are the entry point parameters. The buttons are usually the controls. So the parameters are what controls the codes behavior. Perhaps it sends an email, the email is the button.
Figure out how to turn it off or when printing or engine stops. In code look for the termination stage or the job completion.