My experience is that the esim/nano sim works the same overseas as it does in the US. The selected data sim is the hotspot service and the Voice line is just that. If in the Cellular Data setting you have Allow Data Switching, that can occur also. The instant you switch or turn off either line, it happens.

Either line service will function exactly the same depending upon where you place it. You do not need to "Add Cellular Plan" to gain service from a physical sim card. Merely place in in your phone and it will register the network then connect.

To be even more confusing, there is a great feature that allows your Voice line to use your Data line as a simulated WiFi network. This permits toll free calls to and from the US for instance on T-Mobile while roaming internationally for WiFi calling. This is known as IMS, or Internet Protocol Multimedia Voice Service. and works well. You can accomplish the same thing by having a hotspot and enabling WiFi calling with cellular turned off. The advantage is you can do it all on one device. There is a specific setup required though.

You seem to have more questions, but this might help somewhat. If you can be specific perhaps I can clarify the system somewhat.