Each portal can only be used on one Airtable base. If you need portal access to multiple bases, you'd need to create a separate portal for each base.
Which tables are available in a portal is determined by the configuration of your users table in Airtable. Have a look at this article to learn how to set it up! Once you have set up your users table, you just need to add the tables in the Tables section. A dedicated menu item for each table in the portal will appear in the portal header.
If you don't see a table you'd like to add to your portal in the "Add table" popup, try the "Add missing tables" button towards the top! This will add linked record fields pointing to the selected table(s) to your users table and add the table(s) to the portal!
You can choose which tables are shown to different types of users as explained in this article.
You can also choose a lookup field that displays a linked record field as a portal table, but note that this type of portal table does not support record creation.
You can also display data from other tables by including linked record fields in your main table's child form. This way, users can navigate to linked records from other tables with ease.
To remove a table from the portal. Click on the table in the tables list, and then select "Delete" in the bottom left corner.