I’m leaving Apple! I got this phone day of launch and treat it like my son!
I have followed my batteries' capacities with excel spreadsheets - using software tools to monitor their health, capacity vs design capacity on almost a weekly basis for many many years now. (A hobby of mine).
My Samsung Note 3 lost 55% of its design capacity in 6 months of ownership. Thankfully, back then, you could just pop off the plastic back and pop in a new battery. My Nexus 6 did not fare ... much better.
My iPhones (5, 6+, 6s+, 8+, Xs Max, and 11 Pro Max) have all come from the factory with varying amounts of design capacity vs actual capacity. My 6+ actually came from the factory with almost 10% more capacity than design capacity. My 6s+ came from the factory at 96% - and stayed that way for almost 1.5 years. Both my Xs Max and 11 Pro Max came about 100-200mAh over design capacity and stayed 100% after a year of usage.
I'm a light user of my phones. My wife gets the same exact phone that I do, so I have a comparison as she is a very heavy user of her phone. She has to charge her phone twice as often as I do and usually has 2-3x the charge cycles I do after 1-2 years of usage. (She's already passed me 2x in charging cycles on our 11 Pro Max).
Battery health is dictated by how you use your phone, how far you drain it, if you let it get super hot (in the sun), and various other factors.
Even though my wife has 2x the cycles, her battery health is usually within a few % of mine. (Don't leave your phone in a hot car, try to keep it >20%, don't leave it at 100% for long periods of time, etc).
Having observed batteries across different manufacturers, Apple's batteries are usually significantly higher quality than others in my experience.
If I can, I try to keep my batteries between 40-80% these days.
Good luck with wherever your path takes you!