The best way to understand the relationships is to create sample records and export them.
You can then see how each field is populated and related to other records.
buyer_id is the Buyer who manages the Vendor just as user_id is the Salesperson who manages the Customer. The Partner model is used for Individuals and Companies that can be either Customers or Vendors or both.
The ranking fields are managed by Odoo. If you import via a Customers Menu the customer ranking is set and if you import via a Vendors Menu the supplier ranking is set.