Relationships are ALWAYS PK of one table to data field (FK) in a different table. If the FK is also a PK, the relationship is 1-1 which is very rare. The FK can also be part of a multi-field PK. We don't do that very often because usually, we define autonumbers as a PK rather than multi-field unique indexes.
Joins are not the same as relationships, but for a query to be updateable, the "relationships" represented by the join must be valid. You can join any text field to any other text field or any long integer to any other long integer. Doesn't mean the results will make sense, but the query engine will attempt to perform the join as you coded it. You could join StudentName in the student table to TeacherName in the teacher table. If there are any students who are also teachers, you will get one or more records returned but the query will probably not be updateable since it could create a Cartesian Product. You could also get bogus results if the TomJones who is a student is not the same person as the TomJones who is the teacher. Updating, is a whole different situation. Then more stringent rules come into play.