How does a child find its parent?

[Mike Bernhardt]

Reading the section on delegation in the O'Reilly book, I'm confused about
something: The parent is configured to delegate the subdomain to the child
with glue records, etc. But how does the child know who to ask if a host in
the subdomain requests a record in the parent zone? They don't show any
configuration example for that other than making the child a slave for the
parent zone.