Merging screen recordings without resampling cmd+shift+5 is incredible for screen recording!
Splitting a screen recording without resampling the video is easy with quicktime - Edit/Trim the first part and save, then reopen the original recording, Edit/Trim the second part and save. These save super-fast, a testament to simply being copied rather than resampled.
But what if I'd like to merge (concatenate) two screen recordings without resampling?
Using quicktime's Edit/Add Clip to End seems to force a resampling and takes 10x longer to save. Is there a way around it?
There is a command line tool called ffmpeg, which can be installed with any package manager such as brew or macports.
You can use concat option of ffmpeg like this:
ffmpeg -safe 0 -f concat -i listofvideos.txt -c copy output.mov

Contents of listofvideos.txt:
file 'movie1.mov'
file 'movie2.mov'
file 'movie3.mov'

-c copy makes sure the codec is copied and nothing is resampled.
More info about concat can be found here