where is "Soundtracks" in iMovie on my MacBook?

You are correct. iMovie for Mac does not use the word "Soundtracks" like the iPhone does. The soundtrack/theme music is located in the Sound Effects side bar folder as I mentioned.

They are also located in Applications>Control-click on iMovie>Show Package Contents>Resources>iMovie Theme Music.

The theme songs are short, as you mention, so you would need to string together duplicates of a specific theme music selection in your project. You could also scroll through the rest of the Sound Effects folder to see if there are any jingles in there that would be suitable for a specific project.

Also, there are themes for the Trailer feature. You can open a Trailer then do File/Convert Trailer to Movie, that will convert the Trailer to an iMovie project. In the project timeline you will see the track for the theme song that you can copy and paste into any project you want. There are quite a few trailers from where you can crib the music.

Here's a screen shot of a blank Trailer that has been converted to an iMovie project:

There are also interesting titles and effects that you can crib from trailers and put them into your projects.

-- Rich

In the audio sidebar of an iMovie project, click on Sound Effects and scroll all the way to the bottom. You will find the Soundtracks songs there.

-- Rich

Good question. I never paid attention to the number of tracks. iMovie for iOS is a different app than iMovie for Mac. So different features. With iMovie for Mac it is easy to pull in songs from iTunes, where you can keep a large library of music. I don't know know if that is easy or possible with iMovie for iOS, as I have hardly used that app.

i suppose if you wanted to take the time you could export out the tracks to your Mac and build your own library there.

-- Rich