Ask HN: How do you become productive in a new project as a senior developer?
I am 1 month into my new position, same position you're in - new stack & straight into senior role. My tips so far:1) Take over all the admin stuff you can to free up your devs from distraction and pointless tasks. Productivity and morale will immediately go up. My guys were time reporting into 3 different tools (and this is a <10 person startup!), I just started writing a  summary of our standups and told them they didnt have to do it individually any more, it was appreciated and mgmt still get the info they need.
2) Start with infrastructure stuff - how do you deploy, what's your build process, can anyone on the team draw an architecture diagram (mine couldn't)
3) Write tests, it's low risk changes to the codebase so you cant really break anything, but it'll give you the black box overview of how everything hangs together, detail will come in time
4) Accept that there's no way to immediately get to the level of familiarity the guys who built the system will have. I can spend 2 hours digging trying to find how widget x gets its props passed in, or I can ask my teammate who wrote it and immediately known which file and line its on.