iPhone 7 is charging/discharging erratically. Battery is indicated at about 55% of original capacity, but why is charging it gone neurotic?

Battery is indicated at about 55% of original capacity, so I understand its life is shorter (rightly or not), but why is charging and discharge gone neurotic? Sometimes it reports almost no power left, but then after a restart or something, it shows 100%. This has been going on for the past two or three months. OS is v13.x I think, but not the one that was just released. Possibly I'm still on 12.x (phone is off right now, so I can't check.)

Last night, it charged to 100%, but then, using the same cord, it went down to 1% and hasn't gone higher since. The only thing that will wake it is an aftermarket cord, but it won't charge it. (No change in power level, no "lightning" icon.)

Currently it's stuck at 1%, if I use that cord, otherwise it dies. 1% which will at least keep my iPhone "online" till I can get to an Apple store. The one near me doesn't have an opening till Thursday (3 days from now).

But the thing that troubles me, what if it's an OS issue? I might pay $50 for a fresh battery, but not any amount for something that is not about my hardware.

Have tried: hard reboot, soft reboot, 2 apple cords, two aftermarkets (only one of the latter works).