If my iPhone is under warranty period then I can replace my battery with no cost ?

Only if it drops below 80% and is considered a defect.

The difference being you dropped yours. They can tell by the diagnostics. Otherwise it would have been replaced .

AppleCare+ would have covered the damage.

I’m sorry to say this but good luck with that… I had a battery issue with my iPhone XR and I had warranty as well. I guess I’m an idiot because I purchased one of their ridiculously expensive $60 clear cases, you know the ones that are supposed to “protect” your phone, well I had one of those on my brand new phone… well it did not do its job when my baby niece grabbed it and put it down on the coffee table. Even though I bought an apple case my battery started making the screen push up and apple wouldn’t fix it even though I had warranty. Obviously they don’t care because people are going to buy their phones no matter what… Good luck with your warranty… sorry for the story. I hope they treat you better!