I got a lightning to USB connector off Amazon. Supposedly it works with thumb drives. But plug in USB camera, it powers on, but there is no way evidently to show the images/video on the iPhone display. There are many camera microscopes and borescopes that are much cheaper if you don’t get a display. Along comes https://febon.net. He says there is no UVC hardware to interpret the signal correctly, bet Apple could do that in software. He has two solutions. One is a device (needs power) to put between the camera and the iPhone/iPad and his own app to display. Second solution I think is better, cheaper, and neater as there are not as many wires/cables. His second device takes the USB camera and puts it on your local net. via WiFi as a web page. So iPhone/iPad just needs to go to that IP address on your local net. I just ordered, I have not tested them yet but will report here when I do.
1st device is $130, second is $90. A little on the expensive side. I ordered one of each, in separate orders as I found them though different searches and didn’t know how to navigate his webpage. I e-mailed him immediately and he combined the orders and rebated one shipping charge! So he seems to be pretty trustworthy.