Every night.
Even when plugged in over night, the battery is being used. Everything goes through the battery. Each night the iPhone is left plugged in it chews into the battery's life span, nibbling about 10% or more of a loadcycle every night.
Apple stated publicly that the reason they believe iPhone sales were down (their life blood) was because of their 2018 battery discount replacement program.
What incentive does Apple have for making sure your battery lasts forever? Zero. It's suicide to them. That's why they say "it's fine" to leave the iPhone plugged in over night (when in reality it degrades it rapidly).
But don't take my rant for it, do your own li-ion battery charging research- then you'll see the light too.
I think you should take your tin foil hat off.
Batteries degrade in ALL devices. Apple can’t defy science.