Higher user rights without special group

Hello everyone,

unfortunately I have found absolutely nothing suitable.
I have a user who can always see and open all folders in the sharepoint. The user cannot change data in the folders.
I am not aware of any group or role that the user should have in contrast to all other users.
Is there a way to display the group memberships of AzureAD and Sharepoint users?
(I can't find any differences in the normal interfaces).
Can rights come via Exchange Online?
Deputy regulations? If so, where else can I look.
In the meantime, I have even removed all permissions from this sharepoint folder and access is still possible for this user.

Many thanks for your support