Question about iPhone XS battery capacity

Miksal wrote:

Also, is there anything I can do to make the battery last longer? I've heard that if you let your phone get below 20%, it can damage the battery. Is that correct?

I wouldn't say that letting it get below 20% would "damage" it, but avoiding the extremes can theoretically extend the longevity of a lithium rechargeable battery. All use of a battery will lower the capacity to some degree, but the extremes of the state of charge are supposedly where that loss will be greater.

The problem with trying to do that is that it's extremely difficult to restrict the state of charge range manually (some battery powered devices can be programmed to do that) and then you're left with a lower run time by doing so.

Your battery health may be a bit lower than the interpolated design spec (500 cycles to 80%), but there are all sorts of reasons why, including randomness.

Batteries - Maximizing Performance - Apple

If you want your battery to last longer, turn your screen brightness down. Everything else is minor compared to screen brightness.