I want to make some video files together as one file.

Wondershare video converter lets you clip and join videos, and it works amazingly. Not sure if there is a mac version but you could use a windows emulator or bootcamp.

I would start with Handbrake to make sure all the videos are the same size and format. Then just use Quicktime to add a clip to the end.

I am generally not on the Mac side of the house, so I may not be the best to make those recommendations.
I have used several tools for this very reason over the years.
DVDFAB does a nice job.
Nero has great capability.

iMovie is easy enough, if you can’t afford it borrow a copy by torrenting and then remove it after.