Does it use it in the background, though? I've always double-tapped the power button to open Apple Pay before trying to read the phone. Likewise, when I had an S7 Edge, I opened Google Pay before attempting.
Yes. On both iOS and Android, the wallet should open on its own when the phone detects a payment terminal. You don't have to double-tap the power button.
In my experience, this is definitely not the case. Multiple terminals at multiple stores have not activated Apple Pay on my iPhone X. I've always had to double-tap the button.