If you bought it new, you have a one year warranty. They will check your serial number and will know when the phone was first used. Battery is covered, however Apple will check to see if it is having problems. I usually take my phone to the apple store when I am near the end of my warranty and have them check it out for issues. I’ve almost always received a new battery. When I have taken it in for actual issues, I’ve always received a new battery.
Some issues I have had is that I my phone would indicate I have 25% (or 15% or whatever it was) of my charge left and then all of the sudden it would turn off because there was no charge left.
I’ve found the apple store to be very helpful. Even if they say your battery is ok, make sure they record your visit and go back again before your warranty expires. They usually don’t say no twice.