All lithium-ion battery cells will degrade over time. However, you can do some things to help slow this process down, such as not letting your phone completely die (this is unhealthy for the battery) and not charging to 100% too often. iPhone with the latest iOS have a "optimised battery charging" setting under battery health, which learns your daily routines and tries not to charge your phone past 80% until it knows/ thinks you will need it. This is the best option to preserve your battery.
The fast charger will not degrade the battery any more than a normal charger. I have been using one for nearly two years now, and my battery health is at 87% (which is still quite high).