ffmpeg - map a video to a placeholder? (rectangle/square)
You would use the template below,

ffmpeg -i base.mp4 -i overlay.mp4 -filter_complex "[1]scale=WxH[o];[0][o]overlay=x:y" out.mp4


So, the overlay video is scaled and then placed at (x,y) measured from top-left of base video and the opposite corner is at (x+W, y+H)