How to Deal with Stress Joining FAANG for the First Time?

I wouldn't stress about it too much: you're a new hire who's still getting ramped up. It takes a while to get the ball rolling at senior level: there's a mix of the team/company having the right projects and having the time to build trust between you and the team. If you're worried, just work with your manager on clarifying senior level expectations and syncing with them to see how you're trending. At the 3-6 month mark is when I'd say your manager will start getting clearer reads on your performance and can provide the appopriate feedback.

I think you're doing a great job recognizing and seeking help to address your stress.

You've recognized potential sources for the stress that attributes to your stress level. This is important because this gives us potential clues on how you can begin to tackle your feeling of stress.

I'm hearing that you are stressed due to lack of clarity around expectation for a senior SWE. I agree with @Jonathan C that working with your manager to clarify your senior level expectation will help here.

Specifically, work with your manager on setting up timelines on when the expectation will be clarified. Our brain has a funny way of keeping stress around if it feels uncertain when and how the stress will be resolved. So having a date set on when and how the lack of clarity can be addressed will help.

For imposter syndrome, there is a lot to unpack here. I also struggled with this when I was first promoted to a senior software engineer. Despite evidence and feedback that I deserved the promotion.

To overcome imposter syndrome you will need to get to the roots of why you're feeling this way. One thing that helped me was keeping a journal of compliments from my coworkers on my work. I would go and read the journal of compliments whenever I was feeling like an imposter. Something about reading back past records of acknowledgements from coworkers helped me feel less like an imposter.

Feel free to dm me if you want to chat more about this.

You might be putting the Senior SWE title at FAANG on a pedestal. Give yourself time to acclimate to the new environment, and don't propose any grand plans until ~6 months in. You'll do yourself a favor.

Stop worrying - You're doing great! You got tickets and landed them ahead of schedule. There's not much more that you can do in the first 1-2 months. You have an onboarding buddy too - Your manager has really set yourself up for success.

At the end of the day, maintaining good performance is about building up that relationship with your manager and staying in sync. Follow the advice here: [Masterclass] How To Work Better With Your Engineering Manager

From all the videos on Taro it seems like at the senior levels there’s a lot of leadership and design going on (which held true at my last company), but frankly in the past couple weeks I’ve been here, I’ve only seen engineers across all levels chugging out code as fast as they can (maybe that’s something specific to Apple).

Senior is the level where archetypes start to form, and they really become solidified at Staff. I have actually heard that Apple's engineers are more focused on coding and solving very hard technical problems, so I am not surprised by this. Check out this other thread from an Apple ICT4: "What does a path to staff look like in a coding-heavy environment?"