What if I store 10TB on azure servers and then keep the vm powered off?

Yes, you’ll be charged for persistent storage. No, you won’t be charged for anything on a temp disk, but your data won’t survive a reboot. System disk written “extras” will be added to your “storage” bill.

Questions about products or services pricing are off topic here.

Anyway, yes, you will pay for used storage space even when no one is actively accessing it; the actual price depends on the type and size of the storage. More info here:

https://azure.microsoft.com/en-us/pricing/details/storage/page-blobs/