This is it. I find that establishing a social network is one of the most important things to do in the first few weeks. It makes your work more enjoyable.
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.
Go around introduce myself to my new colleagues, have lunch with them and try to establish a social network as soon as possible.
Again go through your work duties with your supervisor, make sure you have all tools and accesses needed to complete them.
Confirm with HR on company rules, work hours, reporting lines (time sheets, sick-leave report) and if they have any onboarding process.
Quickly look through your work, collect as many questions and uncertainities as possible and ask your colleagues/supervisor.