Try to get enough 1 on 1 video calls with the Senior developers on hand who understand the source code, ask them for it via direct chat on slack if such things are not offered as a necessary onboarding requirement.

Also try to understand every bit of the source code, and make sure to not rock the boat too much on ur first few days always write code that blend in really well with the other pre written codes. You can battle for code changes later on.