How does ApplePay work in third-party browsers on devices not running IOS 18 or above?

Apple has extended ApplePay to third-party browsers.

Pay with Apple Pay in third-party browsers – Apple Support (AU)

The feature is said to be available for IOS 18 and above.

What would be user experience (or a specific error message) if customer tries to scan a barcode and not on IOS 18 or above?

Keen to understand user experience if I introduce that feature.

[Re-Titled By Moderator]