Press the Side button once
If Auto-Lock is enabled: leave the iPhone to lock automatically after the selected time
To set Auto-Lock: Settings → Display & Brightness → Auto-Lock → choose a time
To lock immediately from the Home Screen: press the Side button once
To lock immediately from an app: press the Side button once
To lock using Accessibility (if enabled): Settings → Accessibility → Touch → Back Tap → choose an action like “Lock Screen” (optional)