To lock the screen on an iPhone, simply press the Side button (also known as the power or lock button). This will immediately turn off the display and lock the phone, requiring authentication to unlock it again. If looking for a way to lock the screen to disable touch interaction while still displaying content (e.g., for watching videos or sharing with kids), use the Guided Access feature:

• Go to Settings > Accessibility > Guided Access and enable it.
• Set a passcode for Guided Access.
• When needed, start Guided Access by triple-clicking the Side button (or Home button on older iPhones).
• This locks the screen with touch disabled, and to exit, triple-click again and enter the passcode.

Additionally, you can customize Lock Screen features and auto-lock timing in Settings > Display & Brightness > Auto-Lock to set when the screen locks automatically.