Tootfinder

If you're trying to do the trick that was shared for #nintendoswitch2 where you can use a USB HDMI capture dongle HDMI to USB-C cable to use your iPhone as a camera - don't use the True Visage app mentioned in the video
That will rely on screen sharing mirroring and you'll potentially get an aspect ratio mismatch and see the dynamic island floating in your view
I'd personally suggest the free "Blackmagic Camera" app instead, and then go to:
Settings->Monitor->HDMI Out
Switch from Mirror Display to Video Feed and it will cleanly output your camera. Works for a Lightning to HDMI out adapter for older phones, too
Then just tap on Lense and FPS and pick which combo you like.
Also, if you're not getting any output signal make sure the capture dongle you're using supports 1080p out - not sure the Switch 2 supports anything higher as camera input