I've had a battery electric vehicle (Nissan Leaf) for several years now. All EV owners learn the basics of Lithium batteries.
When you run down a Lithium battery a lot, and then leave in that state, it will (over time) degrade the battery. Other than that, these batteries are the same as other rechargeables - the main enemy is heat. Heavy usage and recharging to 100% create heat, and slowly reduces the useful longevity of the battery. IOW, if you are in 100 degree ambient heat and are heavily using the device, and then charge to 100%, it accelerates long term capacity degradation.
None of this is much worry for watch/iphone/ipad batteries, as they are so much smaller than the battery packs needed to run an automobile. If there is any battery degradation over time, most of us would never notice. By the time we would, we'd have the latest/greatest for replacement.