Avidemux is a very nice app for doing basic trimming/splitting/splicing of video files without transcoding. I've used Quicktime for this task for many years but it sometimes gets a bit cantankerous or outputs files that have issues with some players. Avidemux just works. Granted, I've only tried it with MKVs so far, and not for very long but it's output has been perfect. The app makes it easy to know where the keyframes are and to move to the nearest one to ensure the video isn't corrupted. It even pops up a warning dialogue if you try to save a video that's not been trimmed at a keyframe. The lower part of the interface is not pretty but it's much more useful than Quicktime's equivalent. Some minor things are seemingly trivial but add up to reduced friction—like the save dialogue being prepopulated with the original filename with '_edit' appended. QT just offers 'Untitled.mp4', so I have to copy the name from the original file, paste that in and type something like 'trimmed' on the end.
If you want to join several clips in Avidemux, you just select all files and open with Avidemux and they are automatically placed in sequence (if compatible). Doing that in QT results in a separate tab for each clip, meaning you then have to manually select, copy and paste which is a bitch. To be fair, in QT you can open one file, then go to Edit > Add Clip to End and add several clips to the end of the already open one but it's still more friction than with Avidemux. Unlike a lot of Linux originating software, there's a Mac .dmg for the app, which is necessary if you're a plonker like me.
'Avidemux is a free and open-source software application for non-linear video editing and transcoding multimedia files. The developers intend it as "a simple tool for simple video processing tasks" and to allow users "to do elementary things in a very straightforward way"'