How do the covenants with Moses and David relate to the Abrahamic covenant?
The covenant of law made with Moses and the covenant of a royal family made with David are not separate from God's covenant with Abraham; they came under it and are wrapped up in that Abrahamic covenant. The Abrahamic covenant is the foundational covenant to which these later covenants relate.
Les explains that the covenant mentioned in Isaiah—'as for me, this is my covenant with them'—goes back to Abraham (s247–s248). He notes that other covenants came under the Abrahamic covenant: God made the covenant of law with Moses, and He made the covenant of a royal family with David, but they are all wrapped up in that Abrahamic covenant (s249–s254). The surrounding context also quotes God's promise that His Spirit and words will not depart from Israel's mouth or the mouth of Israel's seed forever (s254–s260). Thus the Mosaic and Davidic covenants are related to the Abrahamic covenant as subordinate parts contained within it, not as independent covenants.