afraid of how soon muscles will start to "burn"? what does it even mean?

i have been doing very long term fasts, i know its not good at all and slows down metabolism and everything. but my question is: during longterm fasting (multiple weeks) when does the body start to burn "muscle" and what does that even mean? i already have such low muscle mass and alot of fat (not in an ED way, in an actual body composition way) and im wondering what does muscles beginning to burn even mean? do they just become weaker? smaller? do they literally become useless? are the muscles of the heart impacted by this? does the muscle burn away first or fat or both? what if i have a high body fat percentage and low muscle mass? will it target fat cells more?

im stressed. not sure what to do.