How to onboard and perform well as senior sde?

Congrats on the new job! For onboarding, I recommend our course here: The Complete Onboarding Guide For Software Engineers

If I were to distill down that entire course to what I personally think is the most important, my advice would be to "Win with quality". It is almost impossible for a newbie to win with velocity and quantity as they're trying to understand how the heck the codebase and overall engineering culture work. What you can do though is to spend an extra couple minutes here and there really polishing your pull requests and going out of your way to make social interactions pleasant (social quality).

After the onboarding course, I highly recommend the mid-level to senior course to understand exact level expectations for a senior engineer: Grow From Mid-Level To Senior Engineer: L4 To L5

I would try to maximize that initial energy you have to talk to as many people as you can to find out what the bottlenecks are in the system. Look for low hanging fruit with high impact.

You'll probably end up talking to people that have been in the company for a while that could be jaded about why a certain thing cannot be done or improved. I would try to question their assumptions to find out exactly how much they are saying is true. It can be easy to defer to someone else as the expert because they've been at the company for a while, but many times, they could have tunnel vision on the problem.

One tip I'll share from the course: be very clear about expectations for your first 1 month, 3 months, and 6 months. Write it down and discuss with your manager!