I'm trying to understand how DNS work and there is something I cannot understad. Let's say I buy the domain example.com and want to use amazingdns as my name server. So in the name registrar I configure the corresponding name servers, lets say ns1.amazingdns.com and ns2.amazingdns.com. Then I configure in amazingdns the record A for example.com, and everything is working.
So, I don't understand why another person couldn't enter in amazingdns and create another record A also for example.com. I don't understand why it would't work. Where is the "connection" that says that example.com is my domain?