How to increase battery health?

How to increase battery health? Pay for a replacement battery. That becomes more of an option when the battery heath drops below ~80%, assuming the iPhone isn’t getting entirely replaced then. Otherwise and as correctly stated above, all batteries inevitably and inexorably degrade and fail, always. You can potentially slow the degradation.

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iPhone Battery and Performance - Apple Support

iPhone Battery Replacement - Official Apple Support

You cannot. Batteries inherently degrade over time from the minute they are assembled at the factory. You cannot reverse the steady decline in capacity over time.