Preface, I always advice just using the device and enjoying it. Too worry about the battery will only limit your usage and thus your enjoyment of the device.
My iPhone 11 Pro Max is still at 100%. I use a 5v 2.4amp charging station most of the time. This is slow charge compared to using the included charger or a mac/mac charger. Charging slower is generally cooler and less stressful for the battery. Note, I use this charging station for convenience not battery health....
That said battery health measured as a % is arbitrary and just an educated guess based on metrics that aren't stable reference points. Its hard to say its an "educated" guess, more of a guess based on hardware knowledge and experience.
Keep in mind this is just Apples way of having more disclosure for hardware throttling in a way the consumer can easily read. As batteries get old there internal resistance increases. As the resistance increases in a battery, heavy loads can cause the battery voltage to dip more and more until eventually it will dip below the shut down threshold before its useable energy is expended (ex phone shuts down at 20%). Throttling the hardware performance will reduce higher current loads and keep the voltage from dropping.
If you ever plugged in a heavy duty appliance and/or tool and the lights in the room dim momentarily when it turns your visually seeing the voltage drop due to energies inability to more/flow fast enough due to resistance somewhere (wire gauge, loose connection in the wall/panel box, etc). This manufactured brown out isn't healthy for electronics and/or batteries (lithium).
We dont know Apples method of obtaining battery health. There are a dozen(s) metrics (impedance, conductance, initial capacity, cycle count, load/temp, etc etc etc) which can be used independently and/or combined.
Your usage and temp conditions the phone is used in can positively or negatively effect battery health when its captured in arbitrary 1% slices. For example if only impedance was used then using/charging your phone at 30 degrees F for extended periods of time will actually degraded battery health and a battery meter will greatly exaggerate the degradation because of higher internal resistance. However if only cycle count is used than using/charging your phone in 30 degree weather will have no effect on the battery at all. Accurately combining those metrics can never result in 100% accuracy. Hence why I said its arbitrary and just an educated guess.
TL;DR - I know its difficult but just ignore battery health unit the warranty is about to expire so you can check if they should replace it.
Side note, many NON OEM batteries are programmed to always read 100%. This is my iPhone 6S with a 2 year old battery that I replaced.
View attachment 891717
I can say from experience this isn't a good thing. It last 3-4 hours with light usage and a heavy load like a AR kit usage will make the phone HOT and make it shut down at ~30%. Its useless, I would gladly take some throttling in this case.