Feeling Overwhelmed and Stressed at my New Job

I have joined a big UK bank as a fullstack developer. Now I have 1 yr of experience mainly as a backend dev in ExpressJS framework.

My current role mostly focuses on springboot + react.

I haven't received any KT and was just given my first bug fix which took like 4 days to fix.

I am feeling quite overwhelmed as I am having to figure out everything by myself when it comes to the codebase. The codebase is hell. I feel like if I ask toao many doubts to my seniors it will leave a bad impression.

It has been 3 weeks since my joining(onboarding and code setup took like 1 week). I am very stressed about this new job. 2-3 leads r quite helpful though.

I have one senior who seems quite strict. He expected me to know everything on my first task itself.

What do I do from here. I would love to hear from some senior developers here and get some advice.