The portal is intended to show records connected to the user's table. This is the default behavior. 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
Views section
Select the appropriate View
Under Filter and Sort, set Choose Records to Display to Personal records only (linked to the logged in user)
This option will display only those records that are linked to the specific user. The Show all records option removes the user-link requirement; it does not bypass other active filters.
You can choose which tables, views and external links are shown to different types of users as explained in this article.
💡Tip
If you want to impersonate one of your users to check which records are visible to them, you can test it by setting the login details for that user's record.