How to share video from a Unity camera on a video call?

Most of the 3D WebView packages work with existing video call websites if you enable the device's camera and microphone like described here. However, 3D WebView doesn't include the ability to share video from a Unity Camera on a video call. The browser engines used by 3D WebView are only able to access hardware camera devices listed by the operating system, such as a built-in camera or USB webcam. So, in order to share a video from a Unity Camera on a video call, you would need to create a custom video call website and then use C# scripts to send the video from the Unity camera to the website over WebRTC (e.g. using Unity's WebRTC package). Please note that this approach won't work with existing video call websites, which can only access cameras listed by the system.