How do i build the most muscle while fasting?!

You don't.

If you don't eat surplus calories & at least 2 grams of protein per one kg of weight, you can't build muscle mass.

On the contrary, fasting and working out will have a catabolic effect; You will start losing muscle mass if you don't eat enough & work out rigorously.

As long as you work your muscles they will be less likely to be used for energy. However I know that it is not recommended to workout vigorously while fasting, so I'd suggest light walking daily instead.

you can't avoid it, but try to exercise during these days in order to reduce your muscle mass loss

I'm not sure it can be avoided if you fast, keep active.