When you add tables or queries to a record source and you don't see a join, Access couldn't find a relationship, or it couldn't find two fields of the same name and data type, where one of the fields was also a primary key. When that happens, you can add a join manually.
Just remember the join has to be logical, meaning it should give you valid data, and the fields you join must have matching or compatible data types. For example, you can join Number and Currency fields, but not Number and Text fields. If you try to join fields with incompatible data types, you'll see an error message.
Add the tables or queries to your record source. In the picture, you can see two fields of the same name. If you do the practice, you'll see they're both foreign key fields, so Access can't automatically join them, but you can.
To create the join, drag one of the matching fields to the other. Access places the field you drag on the left side of the join. If you make a mistake, just click the join line and press DELETE.