Soldato
That’s not correct - certainly for Dahua and Hikvision the devices work just fine as the recorder passes the data to the app like any other connected device. If you have the displays you can even see the surveillance cameras on the room displays.Yes, the ports on the back of an NVR will do PoE.
If you have a PoE doorbell cam, and its ONVIF compatible you could connect it to the back of the NVR, however the app for the doorbell cam wouldn't work as the NVR wouldn't pass through the ports etc..
You could connect the doorbell cam to the network and get power from elsewhere, and then still ONVIF it to the NVR so you have recording on that as well.