The YouTube link below goes into putting this type of system together. Of course, your camera must have a non-4K HDMI output (you might need another adapter cable to get from your camera micro or mini-HDMI into the HDMI capture device which uses a standard sized HDMI input connector). I've been looking into this myself but haven't yet tried it, so this is at an experimental stage.
Not all phones are capable of producing a display from an external USB input, and you'll have to find an app that makes this work (e.g., USB Camera Pro- paid version Endoscope HD Camera and others ) if your phone is capable. I think you need an HDMI capture device (link below) which outputs using a USB type A male connection (you also need a USB type-A to USB type-C cable and your phone must have the newer USB type-C connector).
