
There will be an error message above the SharePoint site when your storage contents exceed the SharePoint Storage Limit like the following picture shows.
These articles will help you learn more about SharePoint Storage Limit:
https://learn.microsoft.com/en-us/office365/servicedescriptions/sharepoint-online-service-description/sharepoint-online-limits
https://www.spguides.com/sharepoint-online-storage-limits/
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