My battery health is decreasing day by day

Battery health is a measure of how much you use the phone and what you use it for. It is not a measure of how long you have owned it. All you have to do to take good care is to not let the state of charge drop below 20% routinely, never let it go to zero without charging it immediately, and charge it overnight, every night with Battery Optimization turned on. There is nothing further that you can do that will make any measurable difference in battery health.

Here is a lot more information—>When to charge your iPhone or iPad | Communities

Because batteries are consumable objects that lose capacity as they are used. The battery in an iPhone is designed to remain above 80% capacity for 500 full charge cycles. Each time you charge it the battery will lose a small amount of capacity. It’s capacity will drop about 1% for each 25 full charge cycles. A full charge cycle is charging from 0 to 100%, or any combination that adds up to 100%, such as 50% to 100% twice or 50% to 75% four times.

You need not worry until it is at 80% of capacity. Check these links for battery tips...

iPhone Battery and Performance - Apple Support

https://www.apple.com/batteries/maximizing-performance/

Do you and your friend use and charge your phones the exactly the same way and times? No, of course you don't. You may use and charge your phone more, which would mean your battery will lose charge faster.