how to split an mp4 into separate clips?
Here's a tutorial I just made to solve the problem. No pro subscriptions, no nonsense. 

QuickTime Export Split Clip Tutorial: https://www.youtube.com/watch?v=g7MgVBGfFCo

Essentially, you just use the shortcut to split the clip, move the split over to a new file, and save the new file. Quick and easy. Split up an over hour long video into 13 videos or so [1] in 15 minutes. 

[1] https://www.youtube.com/playlist?list=PL3z1TiLmRFcwBvXbeSmXhYbouKfBKZyZv