so after I tap on "iCloud Backup" I just turn my iPhone off?

No.

If you want to backup immediately, tap "Backup Now" after you tap iCloud Backup. See items 3 and 4 in the top section of How to back up your iPhone, iPad, and iPod touch with iCloud - Apple Support

If you want it to backup automatically while charging, now and in the future, see the "Automatically back up with iCloud Backup" section of How to back up your iPhone, iPad, and iPod touch with iCloud - Apple Support

You don't turn the phone off. Your phone will backup while it is charging, if it is connected to wi-fi and if it is locked. You can lock it with the right side button or do what I do and let it lock itself automatically based on the time selected in Settings > Display & Brightness > Auot-Lock.