Hello @Danielle Hogan
Thanks for reaching out!
When a new user is added to Azure Active Directory (Azure AD), the user account information is sent to the SharePoint directory store and the UPA sync process creates a profile in the User Profile Application based on a predetermined set of attributes. Once the profile has been created, any modifications to these attributes will be synced as part of regularly scheduled sync process.
Typically, user profiles are created automatically for all accounts that are created in Microsoft 365. For organizations that have a Microsoft 365 Education subscription, user profiles are not created for new accounts by default. The user must access SharePoint once, at which time a basic stub profile will be created for the user account. The stub profile will be updated with all remaining data as part of the sync process.
For the steps to delete a user in the Microsoft 365 admin center, see Delete a user from your organization.
After you delete a user, a series of jobs will remove the user from SharePoint. After the next incremental profile import job, the user (or users) will be marked as deleted, the user's profile page will be deleted, and the user's OneDrive will be marked for deletion by the MySite cleanup job.
Also, please refer these links to know about OneDrive retention and deletion:
OneDrive Retention and Deletion: OneDrive retention and deletion - SharePoint in Microsoft 365 | Microsoft Learn
Set OneDrive Retention for Deleted Users- Set the OneDrive retention for deleted users - SharePoint in Microsoft 365 | Microsoft Learn
Hope this helps.
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