Custom domain of APIM and cname setting

Question

We just configure custom domain in APIM and IT department also adds CNAME for us.
The setting is like this
mycompany.com CNAME myapim.azure-api.net

It works fine when we go to mycompany.com via browser and it shows developer portal.
The nslookup result is here.

What interests me is, why does this CNAME work ?
I think it's supposed to map to portal ( myapim.developer.azure-api.net ) instead of gateway url, but surprisingly it works.

Another question is from this article.
I know APIM would have both private and public ip when established in vnet.
I'm wondering if every service of APIM has their own private/public ip ?
The services I mention here are

There are a number of endpoints to which you can assign a custom domain name. Currently, the following endpoints are available:
Gateway (default is: <apim-service-name>.azure-api.net),
Developer portal (legacy) (default is: <apim-service-name>.portal.azure-api.net),
Developer portal (default is: <apim-service-name>.developer.azure-api.net).
Management (default is: <apim-service-name>.management.azure-api.net),
SCM (default is: <apim-service-name>.scm.azure-api.net),

Thank you

Answer 1

While the CNAME record allows DNS queries to resolve to the IP of the APIM hostname, you still need to setup a Custom Domain on APIM for everything to work.

In your case, it is likely the domain - mycompany.com - has been setup as a custom domain for myapim.developer.azure-api.net, hence the behavior you observe. APIM runs all of these services on the same host (sharing the same IPs) and leverage the Host header to decide which service a request is meant for.

This why though the CNAME record is for myapim.azure-api.net, the IP resolved is still pointing to the same instance but the configuration in APIM makes the request to serve the developer portal instead.