So GoPro MUST breakup a long video into "chapters" (ex. 4GB chapters in Hero7 Black). Fine. How do I re-assemble the chapters into a single, longer video in either GoPro Quick of Fusion Studeo?
In https://gopro.com/help/articles/question_answer/GoPro-Camera-File-Chaptering-Information the "solution" seems to suggest that re-assembling the chapters is quick and easy. So far, with a bunch of searching, I have found it impossible.
You need a proper video editor. Quik is designed to take small parts of clips and sync to music. This will not help yo accomplish what you want. Fusion Studio is for stitching 360 spherical videos, again, not what you want.
Here are some free options from simplest with the least amount of options to a full blown professional video editor.
If you have Windows, the built in video editor is simple and should work in most cases
If you have a Mac, then iMovie works well
If your computer is not very powerful (video editing is some of the most stressful (for the computer) processing you can do), then VideoProc will customize itself to match your system resources.
VSDC video editor is a program designed to work seamlessly with GoPro HEVC video
DaVinci Resolve is a professional editor that is used by Hollywood to make movies. It can seem complex at first, but after watching a few "how to" videos on YouTube, you should be able to figure out the basics enough to create your own videos.
Other free video editors are: