In order of your questions:

• Your body essentially eats them. Muscle fibers require a ton of resources to maintain, and your body is evolved for efficient use of scarce resources. If you don’t use them, your body gets rid of them.

• While it happens pretty quickly, a week or so after working out, provided you’ve been eating and sleeping, you won’t be noticeably weaker. A good rule of thumb is after about 2 weeks of not doing a lift, drop the weight by 10%.

• See the first bullet for the next two questions.

• Barring unusual circumstances, it’s easier to regain muscle than it was to get it in the first place.

Edit: To elaborate a bit on that first bullet, one pound of muscle takes, conservatively, about 140-150 calories per day to maintain. while that may not sound like a lot, in the environment our species evolved in, even 10 pounds of muscles is a good half of what you could expect to scavenge. For comparison, a pound of fat takes about 40-50 calories per day. To give you a sense of what that entails, I'm 6'4 at about 205lb, in the 85th-95th percentile by most strength metrics, though nowhere close to anyone who does it professionally. My base metabolic rate - that is, the amount I'd need to maintain weight in a coma, is well over 2,000 calories. To gain weight at a reasonable pace, with a caloric surplus of ~200-300 calories/day, I need to eat around 3200 calories per day. That is an insane amount of food for a species that evolved in subsistence conditions (that many people still live under), and something that takes serious effort (and money) to maintain in a healthy way.

TLDR muscle is crazy expensive, energy-wise.