Recording command without webcam:
ffmpeg -f alsa -i pulse -f x11grab -framerate 60 -video_size 1600x900 -i :5.0 -filter_complex '[1:0]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih), pad=1280:720:(1280-iw*min(1280/iw\,720/ih))/2:(720-ih*min(1280/iw\,720/ih))/2,setpts=PTS-STARTPTS[main]' -map '[main]' -map 0:a -c:a libmp3lame -c:v libx264 -preset ultrafast -crf 18 -y -threads 0 regular.mkv

With webcam overlay:
ffmpeg -f alsa -i pulse -f x11grab -framerate 60 -video_size 1600x900 -i :5.0 -f v4l2 -video_size 176x144 -framerate 15 -i /dev/video0 -filter_complex '[1:0]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih), pad=1280:720:(1280-iw*min(1280/iw\,720/ih))/2:(720-ih*min(1280/iw\,720/ih))/2,setpts=PTS-STARTPTS[main];[2:0]setpts=PTS-STARTPTS[cam];[main][cam]overlay=W-w:H-h[out]' -map '[out]' -map 0:a -c:a libmp3lame -c:v libx264 -preset ultrafast -crf 18 -y -threads 0 webcam.mkv