Press the Side button and either Volume button at the same time to capture the screen
Open the screenshot thumbnail to keep it on screen briefly
Use Guided Access to lock the iPhone to one app and prevent screen changes
Go to Settings
Tap Accessibility
Tap Guided Access
Turn on Guided Access
Open the app you want to freeze on screen
Triple-click the Side button
Tap Start to freeze the screen within that app
To exit, triple-click the Side button again and enter your passcode