How does dep know the mdm server url?

I'm currently trying to develop an MDM (Mobile Device Management) solution and have encountered some issues:

1. I've set up an MDM server, let's assume the URL is mdm.***.com.
2. I've also registered with Apple Business Manager and assigned the MDM server.
3. Using iOS's Configurator, I added an iPhone device to the Business Manager. After the initial addition, it prompts to erase the iPhone. After restarting, the iPhone interacts with the MDM server and completes registration.

Now, here comes the question: When I was assigning the MDM server in step two, I didn't provide the MDM server's URL, I just uploaded a public key. After erasing the iPhone in step three, how does it know the URL of the MDM server?