Screenshot

Take a screenshot on your iPhone

• Press the side button and the volume up button at the same time.
• Quickly release both buttons.
• After you take a screenshot, a thumbnail temporarily appears in the lower-left corner of your screen. Tap the thumbnail to open it or swipe left to dismiss it.

Take a screenshot on your iPhone - Apple Support (IN)

If you want to use Assistive touch for Screenshot. You may assign

Settings > Accessibility > Touch > Assistive Touch

Assign Custom Actions

I have assigned Double tap for Screenshot. The choice is yours.