Well, I finally found how the customers are linked to their accounts. In the ir.property table there are two fields that establishes their connection.

The 'value_reference' field has data that references the account table and the record id like this: 'account.account,12'

The 'res_id' field has data that references the res_partner table and record id like this: 'res.partner, 18'

what this does is use the account with an id of 12 and associates that account with the customer that has an id of 18

If you activate developer mode and mouse over the field, it will give you the field name.