Hi Ashish,
In Mendix there is no foreign key concept (of course there is no direct foreign key)
Whenever you create an entity, the entity will have one more extra attribute named ID created by Mendix. whenever you create a new object for that entity, it creates one random GUID number and assign it to ID attribute. and this acts as Primary Key for your table.
Now if you create an association, it creates one more table.
So in your scenario in first case It has 3 entities Person, PersonAddress and PersonAddress_Person (name of the Assocation).
so here 3rd table contains only 2 rows, Person_Id and PersonAddress_Id and it stores the GUIDs of respective entities and hence it establishes the association.
So when you delete the Assocation, it actually deletes the table PersonAddress_Person but the GUIDs of Person are still same in Person table.
Now if you created one more association to PersonEmailAddress , it creates one more table called PersonEmailAddress_Person and again stores GUIDs of the respective entities.
Since Person GUIDs are not changed, it uses the same GUIDs of Person here.
Hope this is clarified.
For more details how this works follow below Docs
https://docs.mendix.com/refguide/associations/