For me the problem was the Cable.
So..
MacBook Pro 15" 2018 Model, Catalina 10.15.6 (latest version), Fuji X-T3, USB-C cable in to the Fuji and USB-A (the wide rectangular one) in to a dongle from my Mac.
When I opened my Cam app(s) Zoom, Teams, WebEx and choose the Fuji Cam option for video input I would get the default Fuji Webcam image background but no live video. I was pulling my hair out trying to get it to work, double checked all the camera settings, updated the Zoom, Teams, etc software... nothing.
Then I tried removing the dongle.. and using a USB-C to USB-C cable directly in to the Mac Book. BAM! works perfectly now.
Here's a list of tested USB cables from the NYT... https://www.nytimes.com/wirecutter/reviews/best-usb-c-cables/#fast-charging-and-data-transfer-for-laptops I assume any of these will work well.
Hope this helps.