Streaming screen capture with camera at corner and playback with microphone input. Assuming first graphic card with kms support, camera available and pulseaudio.

live.sh

#!/bin/sh

#required for kmsgrab
setcap cap_sys_admin+ep /usr/bin/ffmpeg

#specify streamurl provided by streaming platform
#streamurl='rtmp://a.rtmp.youtube.com/live2/xxxx-xxxx-xxxx-xxxx'
streamurl=

#specify loopback pulseaudio device
#loopback=alsa_output.pci-0000_00_1f.3.analog-stereo.monitor
loopback=

#specify microphone pulseaudio device
#microphone=alsa_input.pci-0000_00_1f.3.analog-stereo
microphone=

pactl load-module module-null-sink sink_name=mix
pactl load-module module-loopback source=$loopback sink=mix
pactl load-module module-loopback source=$microphone sink=mix
ffmpeg -thread_queue_size 512 -f kmsgrab -i - -thread_queue_size 512 -f v4l2 -i /dev/video0 -thread_queue_size 512 -f pulse -ac 2 -i mix.monitor -vaapi_device /dev/dri/renderD128 -filter_complex "[0:v]hwmap=derive_device=vaapi,hwdownload[screen];[screen][1:v]overlay=W-w:H-h,hwupload,scale_vaapi=w=1280:h=720:format=nv12" -c:v h264_vaapi -c:a libfdk_aac -profile:a aac_he_v2 -f flv $streamurl
pactl unload-module module-loopback
pactl unload-module module-null-sink