Your battary required to be replaced and it will start working Fine.

It hasn't died with a high % showing in quite a while and when they tested it at the Apple Store, it was still "good". They also told me that apps use more power when they start up, so I should leave them open. I talked to someone who repairs phones and she said it should be charged before it drops to 20% to get the best battery life.

This is (and has been for several years) the correct way to use an iPhone/iPad. iOS does a better job of managing power and data usage w/ apps in the background than you can manually.

I'll occasionally manually close an app that has weirded out but otherwise leave them running. Not counting iOS updates I haven't rebooted my iphone X in several months. Its easily the most stable and reliable smartphone I've ever owned (after some 2018 iOS patches...)

I'd hazard the guess that the other equipment you refer to was not powered on for all 24 hours of the day, often with programs running in the background.