Hi,
Thank you for posting your query.
Default permission levels are predefined sets of permissions that you can assign to individual users, groups of users, or security groups, based on the functional requirements of the users and on security considerations. SharePoint Server permission levels are defined at the site collection level and are inherited from the parent object by default.
Go to this link for your reference and other troubleshooting procedures https://learn.microsoft.com/en-us/sharepoint/sites/user-permissions-and-permission-levels
https://learn.microsoft.com/en-us/sharepointmigration/understanding-permissions-when-migrating
-----------------------------------------------------------------------------------------------------------------------------
If the answer is helpful kindly click "Accept as Answer" and up vote it.