I went back to the store and got more clarification.
My plan is Y40/month:
5GB anywhere (not HK/Macau/Taiwan) data + 5GB Sichuan-only data + 2GB data that will start next month (also anywhere data)
So if you don't live in Sichuan it's 7GB only. If you are in Sichuan, it uses the 5GB Sichuan-only data first. You should ideally sign up in the province that you live the most in.
Everything is 5G but if your phone doesn't support 5G, it will just use 4G.
If I want to cancel the plan it's Y60 before one year and no cancellation fee after that. I don't really want to go through this process again anytime soon but perhaps will change it to BJ/SH after a year if I feel like it. In specific, this would involve cancelling the line and opening a new one. I'm not sure if you can keep the phone number if you do that, but I'll cross the bridge when I get there.