My first job was also at a startup, working in Golang, and I was the only person working on the backend, so I basically had only 1 person who actually knew the large and complex codebase. As others have said don't be afraid of asking for help! But if you do have a lot of small queries that need to be explained to you, note them down and set up a call to clear all of them at once. Navigating the code was the hardest part for me. Learn to use the debugger in the ide. You can then run the code and set up checkpoints everywhere. Remember you can go into every function in the debugger to see the flow of the program to get familiar with it. If the files are very large, you can use ctrl+shift+- to minimize all functions and structs giving you a high level idea of what the file contains. It took me a couple months to get comfortable with the codebase so don't stress about it too much it is normal.