Apple has added Apple Pay support to third-party browsers in recent betas of iOS 16 and iPadOS 16.
The added support, spotted by MacRumors contributor Steve Moser, marks a change from iOS 15 and iPadOS 15 and earlier, where in-browser Apple Pay is exclusively available in Safari. Moser found that Apple Pay is available in Microsoft Edge and Google Chrome as of iOS 16 developer beta 4, and other users have noticed Apple Pay support in Mozilla Firefox.
Moser noted that Apple Pay support continues to be unavailable on the macOS versions of Edge and Chrome, presumably since they do not use WebKit, Apple's browser engine that is mandatory for third-party browsers on iOS and iPadOS. This suggests that Apple Pay support is dependent on WebKit and this is the location of any changes Apple has made to expand support for Apple Pay.
It is not clear when Apple made the change, as some users spotted Apple Pay support in third-party browsers as early as developer beta 2, while others only saw support added with developer beta 3.
Source: Macrumors