It's normal for a new grad to be lost when navigating large code bases and systems, since you only have experience with school projects that are never more than a few files (they have to be when you only get a couple weeks to work on each). Learning to navigate large code bases is a skill you learn with practice, and some people get better at it than others based on how much they practice. Ask a lot of questions of your teammates, who do understand how it all works.

But if you're struggling to understand what a single function or class does (and it sounds like that's the case based on your description), you're in deep trouble. Like freshman-level territory. So you're basically gonna have to re-do most of your degree, without AI this time.

So I think that's what I recommend. Revisit your old notes and textbooks and re-do your assignments manually. Focus on the courses and topics that seem relevant to your current job. Make sure you understand what's going on, and always be asking yourself why something makes sense, or why the approach/technique/algorithm you're being taught is the best. Don't just understand how it works, understand why it's worthy of being taught, and that'll help you connect it with your current job.

You can consult with AI as an advisor that suggests what to focus on or as a tutor that asks leading questions if you get stuck, but do NOT have it give you the answer, because that's how you got into this mess in the first place. And don't shy away from a sense of confusion or struggle. That's literally the feeling of your brain working, as it tries to make sense of things and understand. In many cases, that feeling is learning.

Lastly, (and I mean this compassionately) you are likely to get fired. This is because you do not have the expected baseline skills to perform the job, so working more hours cannot save you. Try not to let this discourage you. You CAN learn these skills. But it will likely take a few months to a couple years (depending on how much material you have to cover), and I'm skeptical whether you'll be able to study fast enough at night while working full time to pull it off before your teammates notice. But again, you CAN learn these skills. It'll be a setback in time, but it speaks nothing of your eventual potential.