Where in the DNS process are NS records used? [duplicate]
Amongst other things, it tells the primary who it should send update notifications to when the zonefile is updated (strictly, when the serial number is bumped).
Edit: this has nothing to do with your 1,2,3 sequence above. Recursive resolution doesn't use the NS records; instead it uses the glue records. You can find more about those on Wikipedia, or in this answer .