The under-display camera is a camera innovation that puts the selfie camera underneath the display. Of course, all cameras today are technically underneath the screen physically, but the innovation here is that the area where the camera is, can also display information.
Only OLED screens can do this as you need to make areas where light can go through a window area into the camera and this can only be achieved with self-emissive type of display technology. The challenge for the under display camera is two-fold.
How much of the area can you allocate for the windows to allow the light to go through [bigger area allows more light to go through thus better camera performance, but then the window area would be more noticeable as if you make bigger windows there will be less space for emissive pixels.
Two approaches are undertaken currently in the market with the Galaxy Fold series. One approach is to maximize window area and thus camera performance but result in a very noticeable window area. The other approach seen in models like the Xiaomi Mix 4 would be to have a smaller window area and essentially no change in the emissive pixel structure, thus making the window area not noticeable at all, but with a sacrifice of camera performance.
This is an area where OTI Lumionics technology can help, to make the window area more transparent for both visible light (camera) and IR light.