My biggest concern is the size/scale of what I'm working on. There are endless holes of methods and implementations, and I have no idea where I end up. For example, I see many classes/methods that basically just join other classes/methods. In my previous roles, where I was either coding just for my own consumption or had full leeway in choosing what and how to build, I knew where everything I did fit.
Just getting my dev environment set up was way over my head. I followed onboarding guides, but it was me typing in commands into my terminal and trusting it - I had no idea what it actually did.
I hear about imposter syndrome a lot but I feel like in those cases they have the skills and not the comfort, whereas I don't have the baseline altogether. I was a good interviewer, DS + algos/Leetcode interviews came naturally to me and I worked hard at them, but that's about it. Before switching I was excited because I felt it would be a great opportunity to learn how these big companies work tech-wise, but now I feel like I was handed the blueprints to the Soyuz spaceship in Braille and told to figure it out. I'm not exaggerating when I say I know 0 about what's going on. My team is extremely accommodating and tells me this is natural, but I feel like they're underestimating how far behind I am.
I'm not terrified about getting fired, but I'd still like to do well in this because it truly interests me. How can I keep myself in check and be honest as to when the "you'll get it with experience" period is over and it's just clear I'm not the right person?
Ask your manager to set explicit expectations. Ask them what you should do if you find yourself struggling with any of those expectations. Take it from there.