iPhone keeps restarting when I turn it off

Well, either iOS is corrupt or your phone has a hardware problem. So the first step is to reinstall iOS—>Restore your iPhone, iPad, or iPod to factory settings - Apple Support. If that fixes it before you restore your backup then it’s software. If that doesn’t fix it make a Genius Bar appointment to have the hardware checked out—>Genius Bar Reservation and Apple Support Options - Apple

Try restoring your backup—>Restore your iPhone, iPad, or iPod touch from a backup - Apple Support. If the problem re-occurs restore to factory settings again and don’t restore a backup.

What you do is you put the screen to sleep; your iPhone is still on. Depending of what iPhone model you have you can then wake the screen by againpressing the sleep/wake button, tapioca the screen, raising up the screen.

You can turn off raise to wake (Settings > Display & Brightness) and tap to wake (Settings > Accessibly > Touch).

IF you really want to turn off your iPhone fully, see steps here: https://support.apple.com/en-ie/guide/iphone/iph841379c3d/ios

How can this issue possibly be related to the battery in any way whatsoever? And how does force-restarting a phone solve the specific problem that the user posted?

Here are some fixes that may work:

1) force restart your iPhone (quickly press vol up then quickly press vol down and then press and hold the power button till the phone shuts off. Hold on to it till you see the Apple logo then release.)

2) there is a problem with your battery. Replacing it at the Apple store might help.

How are you turning it off? On an iPhone 7 you hold the side switch until Slide to power off is displayed, then you do what it says.