This. The answers would be different depending on why, how much, and over what period of time.
Are you really looking to lose muscle, specifically, or just lose weight overall? Is this meant to be temporary, or permanent?
If the goal is making weight for a weigh in, and you’ve got a week to prepare, there’s a lot that can be done, and that involves following a well planned protocol, which would also involve a refeed at the end.
If the goal is specifically to lose muscle mass, and it needs to be done in, say, less than a week, you’d have to take drastic measures, depending on how much muscle needs to be lost.
If the goal is specifically to lose muscle mass, but you have a month or more to do it, the plan would also be different.
I’ll go ahead and answer your first question directly as well: If the goal is significant weight loss in less than a week, the plan would essentially be starvation. Eating a minimal amount, strictly for the purpose of survival, but with plenty of water intake, assuming you’re not trying to lose anything aside from muscle. Again, making weight for a weight class would be different. But in any short term plan for significant weight loss (less than a week), I would STRONGLY advise against any form of cardio. This process can take a significant toll on your heart and cardiovascular system in general, without the added stress of cardio exercise. Adding in cardio is a recipe for a heart attack. Don’t do it.
Anyway. Give us a little bit more information to go on, and you’ll get better advice. The more specific the parameters you present, the better.