Say I have a public server with a service (email, web server, etc) that’s accessible through myservice.example.com, and I would like to restrict that service with a VPN. How do I do that?

I know how to setup a VPN. I know how to use some of the services through that VPN. But see, if I want to use that VPN, I connect my client to that VPN, then I get the subnet of that VPN, say 10.10.100.0, through which I can access the devices by address.

But I see some services offer things like myservice.example.com, and they only work when that VPN is connected. How does that work? Is it just some DNS setting at the domain level or there’s more to it?