Hi, Gary,
iMovie 10 does not have a feature for creating chapters. iMovie 6 and iMovie 9 had the capability of adding chapters (not sure about 7, or 8), but iMovie 10 eliminated that feature. So, if you have a workable iMovie 6 or iMovie 9 version on an older 32 bit Mac, you could import the Mp4 movie into one of them and add chapters there. I think also you can add chapters with version 6 of Garageband (but not the newer version 10). The chapters would need to be added to the exported Mp4 file.
I have seen the app Subler recommended for adding chapters, but have not used it myself. If you want to check it out you can click on their website link here:
https://subler.org/
Another app you might check out for adding chapters is MetaDoctor that is sold in the online app store for about $15.00 for the pro version. However, it has limitations, as a recent poster on this forum has said that it does not work to add independent chapters to a standalone movie. That is, it can add chapters but the only way to read the chapters in the standalone movie is through the MetaDoctor app.
You can add chapters in iDVD itself, if you still have a working iDVD app. The only thing is that iDVD adds the chapters in selected time intervals, like every 2 minutes, 4 minutes, etc. However, you can open the DVD in the free download, Handbrake, and export it out as an Mp4 with chapters.
You might take a look at the following link to a thread on this formum to see if anything in there helps you:
How to transfer old family DVD movies, in… - Apple Community
-- Rich