There are two methods of viewing your OneDrive usage.
The storage usage displayed at the bottom of your M365 sidebar is the storage you are actually using which counts against your quota.
The M365 storage metrics page provides a detailed view of what folders and files are taking up the most storage in your OneDrive. The total storage used may not be the same as reported by the M365 account sidebar due to the way the storage is calculated. For the most accurate report of the OneDrive storage you are using, always refer to the M365 account sidebar.