Your block of storage is cumulative across all your users and domains (it's for your organization as a whole). To prevent disruptions in working when your storage or any other pre-purchased resources runs out, we do not stop any service for any of your users. Instead, we allow you to extend as much as you need (and also shrink back if required) and bill you on a pro-rata basis for the extra consumption in terms of storage, user licenses, and other resources.
This policy came into effect in Sept 2019 with an announcement and several notifications to all our customers. Learn more about notifications sent.
To get visibility into your usage on a more regular basis, you can subscribe to reports here.