The portal is intended to show records connected to the user's table. But it is also possible to show all records from a table, even those that are not connected to the user's table. To limit this for the logged-in user, this should be the setup per table:
Go to Portal Tables
Click on one of the tables
Go to one of the Custom View sections
Set Choose Records to Display > Personal records only (linked to the logged in user)
This option will display only those records that are linked to the specific user. The other option will display every record in the table, regardless of user association.
If you want to impersonate one of your users, you can test it by setting the login details.