Use Quicktime's editing tools to combine multiple video files into a single new video file.
Video: Combining video files in Quicktime
In the QuickTime Player app on your Mac, open the movie you want to add to, then choose View > Show Clips.
Do one of the following:
Insert a movie at the end of the movie: Without selecting anything, choose Edit > Add Clip to End.
Insert a movie at the beginning of the movie: Without selecting anything, press Option, then choose Edit > Add Clip to Beginning.
Insert a movie elsewhere in the movie: Select a clip, then choose Edit > Insert Clip After Selection.
Find and select the movie you want to add to your movie, then click Choose media.
When you’re finished, click Done.