How to enable QR scanning from screenshots on iPhone

All iPhones newer than the iPhone X can scan QR codes from screenshots.

Open your photos app and go to the screenshot containing the QR code.
Long press on the bottom right corner to activate the live text scanning, a yellow circle should appear in the screenshot around the detected QR code.

Tap the yellow circle and you can select to open the URL in safari.

If you dont get the option to either activate live text, or no QR code shows up in the screenshot, you might have to activate the settings to allow this manually.

This is a support article from Apple describing how to enable live text:
https://support.apple.com/en-us/120004

Also make sure to activate QR code scanning and live text scanning in the Camera settings. Go to Settings -> Camera -> tap "Scan QR codes" and "Live text".

Go back to the screenshot in the photos app and start from the top!