How do I turn it off?

You can do this in the Control Center. Pull the Control Center down from the top right corner. Find the Focus Button, just below Screen Rotate Lock and Screen Mirror, tap and deactivate the active profile.

If Do Not Disturb or another Focus profile activates automatically, then a Schedule is active.

To disable or delete this schedule, go to:

Settings → Focus → Select Do Not Disturb → find SET A SCHEDULE → Tap into the time → tap Delete Schedule or just deactivate.

You can read more about how to use Focus here:

Use Focus on your iPhone or iPad - Apple Support

Perfect. Yes, when Do Not Disturb is active, your calls will go straight to Voice Mail, your messages will be silent, and you won't see the red badges on your apps that indicate there are notifications. But all of this can be fine tuned - you can read about that in the link at the bottom of my first answer.

You are welcome, just ask if you are in doubt.

Open Control Center, tap on Focus, turn off the Focus activity.

When done, restart your iPhone.

Regards

Giulio