I'd honestly talk to your teacher directly rather than guess what your school allows. The vagueness is the real problem here - different instructors interpret those policies wildly different ways, and one teacher's "using AI to understand concepts" might be another's academic integrity violation. A quick email asking "is it okay if I use ChatGPT to help me work through this concept?" takes two minutes and saves you a ton of stress. Most teachers appreciate when students ask first instead of getting caught later.
The distinction you're making between understanding and copying is solid thinking, but it's also super easy to blur in practice. When you're using ChatGPT as a tutor, you're relying on it to explain things in a particular way - which shapes how you think about the problem. That's different from, say, asking it to solve a physics problem step-by-step. The gray area is genuine, which is exactly why you need your teacher's take, not what other students are doing.
As for what everyone else does - honestly, it's all over the place. Some students won't touch it, some use it constantly, most are probably doing something in the middle and hoping it's fine. That's not a good guide for your decision though. Your school's integrity policy exists to protect you and make sure your grade actually reflects what you learned, so getting clarity on how AI fits into that is worth the awkward conversation.