how to turn on and off 12 mini iphone

See Restart your iPhone - Apple Support

or

Close an app on your iPhone or iPod touch - Apple Support

depending on which is your question.

Apps are off when you switch to another app. See the following: Switch apps on your iPhone, iPad, or iPod touch - Apple Support

You don't normally need to force close any apps but you can use the following instructions if you ever need to: Close an app on your iPhone or iPod touch - Apple Support

On an iPhone 12, the battery charge % is shown in the Control Center at the top right of the screen. Swipe down from the top right corner of the screen to open Control Center.

See the following instructions to buy iTunes music: Buy music from the iTunes Store - Apple Support

To increase the ring volume, go to Settings > Sounds & Haptics > Ringer & Alerts and slide the blue line to the right..

If it doesn't ring for an incoming call, some things to check:

The ring/silent switch on the left side of the phone. See: https://support.apple.com/en-us/HT208353

Settings > Phone > Silence Unknown Callers: https://support.apple.com/en-us/HT207099

Do Not Disturb: https://support.apple.com/en-us/HT204321

If none of the above, also try restarting: https://support.apple.com/en-us/HT201559

To completely clear an old iPhone after setting up a new one, see: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support