I will answer your questions one by one as below:
Question 1. What does "1 TB per organization plus 10 GB per license purchased" mean for Teams?
It means each tenant has 1TB storage for files. And when you buy a subscription, it will contain a number of purchased licenses( assume as X). So, the storage of your tenant will be (10GB*X + 1TB).
Question 2. If I create a team, does it have the 25T storage by default?
This number is limited by the whole storage of you tenant. If your tenant’s storage is less than 25 TB, The storage for Teams files will also less than 25TB.
Question 3. If I create multiple teams, will they share the storage quota?
Yes.
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