How to maintain my iPhone battery health at 100%?

"To keep" is not possible, since batteries are consumable.

All you can do, after reading carefully the support article that led you here, is listed in these user tips:

Batteries - Maximizing Performance - Apple

and/or

When to charge your iPhone or iPad - Apple Community

Regards

Giulio

Battery Health on a new battery will initially start at 100%. But, after that, the Battery Health only goes in one direction.....down....at an average rate of about 1% a month.