FFmpeg - Overlay one video onto another video?
If you just want a ffmpeg command, try
ffmpeg -i input.mov -i overlay.mov \
-filter_complex "[1:v]setpts=PTS-10/TB[a]; \
[0:v][a]overlay=enable=gte(t\,5):shortest=1[out]" \
-map [out] -map 0:a \
-c:v libx264 -crf 18 -pix_fmt yuv420p \
-c:a copy \
output.mov
This starts the overlay at 5 seconds with the overlaid video start point being 00:15.
setpts=PTS-10/TB is setpts=PTS+(overlay_delay-video_trim_in)/TB
overlay=enable=gte(t\,5) is overlay=enable=gte(t\,overlay_delay)