Just to expand on the answer, opening or importing a video in iMovie won't automatically show the content library in the left sidebar, and the menu items will be greyed out. The import or vid actually has to be made into an actual project. It took me a while to figure this rather (though hardly uniquely) unintuitive part of iMovie.
One would expect to open, say, an Audacity, Powerpoint, Word, etc. file in their respective programs and immediately start messing with it with all the applicable tools, or else the tools would be made operable in an immediately intuitive way. The 'help' for iMovie doesn't tell you this, it just blithely tells you the content library will be found in the left sidebar.
Apple's focus for many years now has been on whatever's best monetizable. iMovies didn't even install when I upgraded to Sierra. It doesn't make money to be helping people with freebie apps, let alone paying people to make the things more useable.