How does Request For Quotation connects with Supplier Quotation

I saw 2 DocTypes, Request For Quotation and Supplier Quotation.

When I create Request For Quotation, I can create Multiple Supplier Quotation.

My confusion was how are they connected in the database. I can’t find any field in the DocType settings which links them. I though find that in 'Connection" tab, they are getting shown. And when I click them, Supplier Quotation in the Connection Tab, I get redirected to the Supplier Quotation with the filler ‘Request For Quotation Equals {RFQ_ID}’ But where is this filter coming from since there is no Request For Quotation field in the DocType.

Am I missing something here?

			"Request for Quotation Item": {
				"doctype": "Supplier Quotation Item",
				"field_map": {"name": "request_for_quotation_item", "parent": "request_for_quotation"},

[Request for Quotation Item].name [Supplier Quotation Item].request_for_quotation_item
[Request for Quotation Item].parent [Supplier Quotation Item].request_for_quotation