I have a normal Google Android phone that i noticed runs out of batteries so quickly even if turned off. So I did a quick experiment.
I removed the battery for a few days. Wouldn't you know it, the charge was exactly as I left it, unlike before. Makes you wonder what exactly that phone is doing using power while supposedly turned off.
I did a big update on my old iPhone. Immediately after, it wouldn’t hold the battery for even half a day. I was forced to upgrade. Guy at phone store said that it’s purposeful, and disgusting. So I don’t do updates now with my new(er) phone