BRAND NEW MACBOOK BATTERY HEALTH DECREASE 5% IN 2 WEEKS!!

Rechargeable batteries can lose some initial charge quickly and the last for a long time at that level before losing capacity, some will hold initial capacity for a long time and then start to lose capacity, some will start slowly losing capacity from day one, and any other possible combinations.  It is just the nature of rechargeable batteries.

I have an M1 MacBook Air that dropped to battery capacity of 90% in the first 6 months and has stayed there for the past 3 years. Just the nature of these types of batteries.

FWIW, don't try to micromanage your battery and just use the Mac.  Just make sure that "Optimize Battery Charging" is turned on. If you reach a point in time where the working time on battery is not long enough to meet your needs, then start to worry or just get the battery replaced.

Apple never provided the Maximum Capacity for batteries before so people don't understand how Apple Lithium batteries work over time. The Full Charge Capacity (FCC) of the battery fluctuates up & down with Apple Lithium Batteries. That fluctuation of the FCC can amount to 5% so that can be completely normal. Here is another post where I explain it with a bit more detail including an example:

Battery Health draining fast - Apple Community

The Apple laptop battery is designed to retain up to 80% of its original capacity at 1000 complete charge cycles.

The one-year warranty includes replacement coverage for a defective battery; however, in your case your laptop's battery has not met this criteria, thus why Apple Support won't replace it.

Now is it possible that this particular MBA's battery is defective and fail to maintain 80% with less charging cycles, yes ... but, AFAIK, Apple will not replace it, under warranty, until it does.

Not sure what you want Apple to explain to you. Lithium-ion batteries are extremely common today. They have numerous advantages over the other battery types. However, they do have two significant downsides ... and those are: 1) If left unused for an extended period of time (months) at a very low charge or completely depleted, they will lose their ability to be recharged, and 2) As I sure you're fully aware of, if physically damaged, they pose a safety risk.