How do you lock the screen on iPhone?

Use passcode to lock iPhone, see this Apple article: Use a passcode with your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT204060

Recommended:

Set Require Passcode -> Immediately.

Also set Auto-Lock -> 30 seconds.

FYI: never forget the passcode.

The screen locks when you press the button on the right side of the phone. It will also lock automatically after the phone has been idle for a time period that you set in Settings/Display & Brightness/Auto-lock. To unlock it swipe up from the bottom of the screen or press the button on the right side of the phone when it is locked.

To enable automatic backup plug in the phone overnight in a location where you have Wi-Fi signal and lock the phone with the side button, or just wait for it to auto-lock (unless you set it to Never).