Video overlay is a technique that displays a foreground video or image on the (usually bigger) background video or image
FFmpeg Command structure for overlay
ffmpeg -i input1 -i input2 -filter_complex overlay=x:y output
FFmpeg Logo in one of corners
FFmpeg command for Logo in top-left corner
ffmpeg -i pair.mp4 -i logo.png -filter_complex overlay pair1.mp4

FFmpeg command for Logo in top-right corner
ffmpeg -i pair.mp4 -i logo.png -filter_complex overlay=W-w pair2.mp4
FFmpeg command to Logo shows in specified moment
ffmpeg -i video_with_timer.mp4 -itsoffset 5 -i logo.png ^
-filter_complex overlay timer_with_logo.mp4