Concatenating videos with the Quicktime Player requires unnecessary decoding/encoding, with takes time and computational resources.

https://trac.ffmpeg.org/wiki/Concatenate

Using ffmpeg

QuickTime Player Version can record movies. If the recording process in interrupted by battery running out, or the computer being closed and put to sleep, the video file is store at the following location: ~/Library/Containers/com.apple.QuickTimePlayerX/Data QuickTime Player Version: 10.4