An external webcam is a video (and possibly audio) source that isn't build into the local computer or connected to it via USB.
On Android Start DroidCam and get the webcam URL.
ffmpeg -re -i http://192.168.XXX.XXX:4747/video -map 0:v -vf format=yuv420p -f v4l2 /dev/video1
See https://theterminallife.com/using-ffmpeg-and-v4l2-loopback-to-play-youtube-videos-as-a-webcam/
A recent version of youtube-dl
may be needed.
Very slow and freezes quickly.
See https://superuser.com/questions/680323/processing-youtube-video-in-ffmpeg - unfortunately not easily possible because ffmpeg does not support live-streaming anymore (?!)