Crop video to square

I want to crop a video to square. I can't figure out how to do this on either my iPhone or my Mac, using either the Photos app or iMovie.

I looked for an app to do this, but all the ones I found either didn't work, didn't do what I want, or tried to extort me into "subscribing" for a ridiculous monthly fee just to do one thing one time. (Don't get me wrong, I am willing to pay for software, but this everything-is-a-subscription thing has gotten totally out of hand).

And in any case, shouldn't I be able to do this with iMovie?

Thanks,

Frank