How can a domain nameserver be a lower domain?

Question

Domain Name System

DNS is known to have a tree structure.
Those. to resolve the domain name smtp.example.com to an ip address we go through the following chain (ns - nameserver):
Root dns server . -> ns of the .com domain. -> ns of the example.com domain. -> ns of the smtp.example.com domain.
How can ns1.example.com be the nameserver of example.com if, in order to resolve ns1.example.com, it is logically necessary to refer to a higher level domain, i.e. to example.com?
In other words:
To resolve example.com, you need to go to ns1.example.com
And to resolve ns1.example.com, you need to go to example.com
The only explanation that comes to mind:
to ns zone .com. there is an entry of type A ns1.example.com - 1.2.3.4
But this does not fit into the tree structure and looks illogical. (in my opinion)
What is wrong in my picture of the world?

Answer 1

Everything is fine in your picture.
ns.example.com 1.2.3.4 are registered on the root DNS of the .com zone, by the registrar.