> FFM isn’t entirely muscle, but what other weight would be shed when losing FFM other than muscle?

I'm not an expert, but I have to imagine that most of it is muscle.

After dramatic weight loss, a person will probably lose some bone - particularly in the lower body - due to decreased loading.

I know body builders sometimes eat extremely high protein diets (more than 1 g/lbs of body weight) and lift quite hard to try to hang on to as much muscle mass as possible. And they still lose some when cutting.

Water weight is a big one, and is part of your FFM. I lost 10lb of water weight in my first 24 hours on tirzepatide.

Some of it is likely bone density as well. You can prevent the bone density and muscle loss with proper diet and exercise, though.