My battery health drop 1 percent every week why??

A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service cover for a defective battery. If it is out of warranty, Apple offers battery service for a charge. Learn more about charge cycles.

From https://support.apple.com/en-gb/HT208387

The help articles above will greater enhance your understanding of how the iPhone battery works.

There is no issue, that is normal

There is many factors to why.