Battery health

See if any of the following support articles can answer your questions:

• Maximizing Battery Life and Lifespan - Apple
• About the battery and performance of iPhone 11 and later - Apple Support
• 21 Tips on How to Improve iPhone Battery Health in 2021 - TechieTechTech

The battery health will decrease as you use it. You can’t go back to 100%. When it gets to 80% or less, get a new battery.