Junior dev trying to pickup existing codebase. How do I do it?

It feels acary as fuck! I am self tought guy who can write working JS, Python and some SQL, but an in depth understanding of the architecture and servers and all that is quite beyond me atm. I've developed and deployed a few web apps in the last couple of years and this is my first 'proper' proramming gig (it's a bit less programming than you'd expect from a developer role actually).

I am expected to pickup an existing project and make a few changes, develop it further etc.

Project was developed single handedly by a guy who left the company and no one else has any experience of Typescript/React which covers 80% of codebase. It uses it's own API and Postgres. There is only 1 other developer at the company who does most stuff in Pascal/Visual basic.

The project I need to work on is not extremely complex, but it's still quite a lot as I'm only learning things.

Any pointers on how should I approach the whole thing? I don't think I can get much technical help from other colleagues though. Do I just power theough a course on Typescript/React first or dive striaght into code and try to figure it out from there?

Should I take some time first to understand GitLab, servers and general architecture first? Or dive into code and get familiar with things that I absolutely need to know to implement changes?

Any pointers greatly appreciated!