I looked at the CUDA code for Leela Chess Zero and found it pretty understandable, though that was back when Leela used a DCNN instead of transformers. DCNN's are fairly simple and are explained in fast.ai videos that I watched a few years ago, so navigating the Leela code wasn't too difficult. Transformers are more complicated and I want to bone up on them, but I haven't managed to spend any time understanding them.

CUDA itself is just a minor departure from C++, so the language itself is no big deal if you've used C++ before. But, if you're trying to get hired programming CUDA, what that really means is they want you implementing AI stuff (unless it's game dev). AI programming is a much wider and deeper subject than CUDA itself, so be ready to spend a bunch of time studying and hacking to come up to speed in that. But if you do, you will be in high demand. As mentioned, the fast.ai videos are a great introduction.

In the case of games, that means 3D graphics which these days is another rabbit hole. I knew a bit about this back in the day, but it is fantastically more sophisticated now and I don't have any idea where to even start.