Proper steps to back up then factory erase iPhone

What’s the support article for properly backing up the iPhone & then factory reset it then sit up the iPhone? I recently installed an app directly from the App Store & even though I deleted it & restarted the phone & even a hard restart the phone battery life has significantly dropped, like a tracker software or something weird. I don’t want to say the App’s name for legal purposes, but it was directly from the App Store.

iPhone 13 Pro Max iOS 15.5