iPhone 15 battery drain fast and battery health down

Don't keep letting it drain down to 20%. That's shortening the battery life significantly. Charge it more often. I keep mine on the charger whenever I'm not using the phone, though I know that's not always possible. And leave Battery Optimization on and let your iPhone learn the best charging level. For more information, see:

Batteries - Maximizing Performance - Apple

Regards.

That seems to me to be quite a few charging cycles for a nine-month-old iPhone. In any case, Battery Health will decline about 1% for every 25 charge cycles, so having your Health down by 11% is in keeping with that number of charge cycles (it's actually a bit better than average). You may need to consider how and how often you're charging your iPhone. If you're regularly letting it drain far down before you charge it, don't. That will just shorten the life of your battery.

Regards.

How many charging cycles is it showing?