Apple Pay: From https://developer.apple.com/videos/play/wwdc2016/703/?time=9...: "You take an Apple Pay server URL, and this URL is provided from Safari. You send this URL to your web server, which then requests the merchant session." -> Seems like Safari only :/
PiP: From https://developer.apple.com/macos/: "And if you use a custom video player, it’s easy to add a Picture in Picture control using the JavaScript presentation mode API." -> Seems like it's possible for 3rd party browsers to implement it!
No so the window.Payment[1] isn't in the right place yet—according to Apple—to be ready for their use. You can read more in [2]. So eventually ApplePay.js would (I presume based on their language) disappear to some extent and all browsers could use the Payments API and Apple wouldn't need their own custom solution.
PiP: From https://developer.apple.com/macos/: "And if you use a custom video player, it’s easy to add a Picture in Picture control using the JavaScript presentation mode API." -> Seems like it's possible for 3rd party browsers to implement it!