Now I don’t know where to put the resulting files under lib/ and shotcut/share.
My variant of shotcut comes out of the snap respository. It’s files are quite isolated, located under /snap/shotcut/59/Shotcut.app/ where everything is a readonly sqashfs-mount.
So I can’t simply copy or symlink stuff to /snap/shotcut/59/Shotcut.app/lib or /snap/shotcut/59/Shotcut.app/share because of the readonly nature of that location.
Is there some other place where shotcut on linux might look for its files and plugins, like ~/.local/ or ~/.config?
I do not think it can work on snap. It only looks in one location relative to its executable. You can try using the portable tar, it is the same build as the snap.