How to stop my iPhone screen from dimming automatically?

To turn auto-brightness on or off:

• In the Settings app, tap Accessibility.
• Tap Display & Text Size.
• Scroll to Auto-Brightness and tap to turn the feature on or off.

Adjust brightness on your iPhone - Apple Support (IN)

Adjust brightness in Control Centre

1. On your iPhone X or later, or your iPad with iOS 12 or iPadOS, swipe down from the top right-hand corner of your display. On your iPhone 8 or earlier, swipe up from the bottom edge of your display.
2. Drag the brightness bar. up or down to adjust the brightness.

Adjust brightness on your iPhone - Apple Support (IN)