How do DNS servers not get hijacked? [duplicate]

Well, what if I create a set of name servers and have them set as someone else's domain?

You can stand up a DNS server and have it serve requests for whatever domain you'd like. That would be a completely pointless exercise, though, because requests for those domains would never hit your server.

What is stopping the domain name from resolving to the lookalike DNS servers I just created ?

Well, as you pointed out, nameserver records are set at the domain registrar. If they're set properly, requests for your domain will only ever be referred to those servers.

What if someone randomly creates two name servers that you created, ns1.peanuthead.com and ns2.peanuthead.com?

You seem to be quite confused. If you are in control of example.com, only you and you alone can create DNS records in that domain. Sure, anyone else can configure their DNS server to be authoritative for that domain, but again, that's pointless as queries for example.com will only ever be directed to the specified nameservers.

What what stops the domain peanuthead.com from querying those name servers created by someone else?

This question is nonsensical, as no one else can create records in your domain, and no one but you can edit the nameserver records at your registrar.