You can only use Apple Pay in Safari. But if you use the standard W3C Payments API, a Chrome browser running on an Android device with suitable security will use Android Pay just like an Apple device running Safari will use Apple Pay.

Apple Pay is Safari’s implementation of the standard. Apple stores your payment information and uses the Secure Enclave for authorization. How could Apple ensure a secure path on non Apple browsers or on non Apple hardware?

What’s the use case where you would use Apple Pay as opposed to your hardware vendors implementation on a non Apple device?

Just a snippet from the standard.

https://www.w3.org/TR/payment-request/

This specification standardizes an API to allow merchants (i.e. web sites selling physical or digital goods) to utilize one or more payment methods with minimal integration. User agents (e.g., browsers) facilitate the payment flow between merchant and user.