The executable for macOS is in
Contents/MacOS/ImageJ-macosx according to macOS conventions. It needs to live there so that double-clicking on the
Fiji.app launches the program as intended. You can certainly launch that executable from the command line, just like with other OSes.
IIRC, one idea we considered a couple of years ago was to put the
ImageJ-macosx in the top-level folder along with the others, and then symlink the
Contents/MacOS version to that, but when we tried it, there was some technical issue preventing it from working as intended. So we gave up on it. But feel free to play with something like that, and if you get it working, we could implement it.