Where did Cain get the people from to build his city which he named after his son?

Welcome to one of the three biggest plot holes in the book of Genesis.

The canonical story has no answer to this, so countless religions have retconned this in Bible fanfic since about the time literacy caught on.

I haven’t seen any retcons that I find terribly persuasive. Middle Ages theologians said Nod wasn’t a literal city, it was endless desert populated by monsters. Origen of Alexandria took the “hell is separation from God” approach and wrote that the city of Nod wasn’t a physical city at all, it was a metaphor for being cut off from God’s Grace.

Roman Catholics believe the word “Nod” comes from “nud” (“to wander”), and so the City of Nod was a metaphor for being cast out and forced to wander—it was a state of being, in other words, rather than a physical place.

Evangelical Protestants say that since Cain was exiled and travelled to the land of Nod with his wife, but the only people who existed to that point lived in the Garden of Eden, Cain must have married and then been exiled with his sister. The city was then built by Cain himself (though how you call a place with three inhabitants—Cain, his nameless sister/wife, and his son Enoch—a “city” is left unanswered.

Progressive Protestant denominations believe the story of Adam and Eve doesn’t describe the creation of all humans, but just a specific lineage of humans, namely the lineage that gave rise to the ancient Israelites. Other lineages of people already existed. Cain met his wife after leaving the Garden and intermarried into a separate lineage; with them, he built the city of Enoch.

Some Anglicans believe the city was built not by Cain, but by his descendants, specifically on the Euphrates in present-day Iraq. The issue of where Cain’s wife came from is left unaddressed.