1 customer can have multiple people
That suggests that you have this one to many relationship (though your field names may be different):
Customer::__pkCustomerID = People::_fkCustomerID
but only 1 of them is a primary contact.
Which would indicate that you also need a one to one relationship between Customer and the record selected as the Primary contact. In order to have two different relationships between the same two tables, you need to add a Tutorial: What are Table Occurrences? of one of the tables in Manage | Database | Relationships. Go to that tab in Manage Database, select People and use the duplicate button (Two plus signs) to make a copy of the People table occurrence. If needed, add a primary key field to People over on the Fields tab so that you can then set up this relationship:
Customer::_fkPrimaryContact = People|primary::__pkPeopleID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
_fkPrimaryContact can be formatted as a drop down list or pop up menu of people ID's and names. This can be a conditional value list of just those People records linked to Customer by the original relationship. Or you can set up a portal to People and put a button in the portal row for selecting one one of the listed People records as the primary. The button would set _fkPrimaryContact to the __pkPeopleID field of the clicked portal row.
That's a lot to work through for someone new to FileMaker and databases so feel free to ask follow up questions where needed.