Apple now offers a native solution for using an iPhone as a webcam. The instructions can be found at https://support.apple.com/en-us/HT213244

Previous instructions are available here:

1. Open camera.html on your phone
2. Click "Capture Back" for back-facing camera
3. Click "Full screen" to make the video feed full-screen. Optionally double tap on the video to completely fill the screen
4. Enable airplane mode on your iPhone to disable push notifications
5. Connect your iPhone to Mac using the lightning cable

1. Download, install, and open OBS Studio
2. Set the right canvas resolution in OBS Studio settings by clicking "Settings" and then "Video". "2436x1125" worked well for iPhone 11 Pro
3. Add "Video Capture Device" as a new source in OBS studio by clicking + under sources. Use any name and select your phone as a device
4. Start Virtual Camera by clicking "Start Virtual Camera"
5. Your phone screen should be mirrored in Zoom, Google Meet, or others if you select "OBS Virtual Camera" as your camera