iOS 18 side button don’t work

Side button has never opened Wallet app. It activates Apple Pay and displays default payment method for use at a merchant’s location.

Apple Wallet app looks like the app icon below. Tap the app to open the app.

None, it’s a secure app with its content in the Secure Element (SE). To access Apple Wallet tap the app.

Technically it’s never opened Apple Wallet. You can access your loyalty or rewards card by tapping the screen when you activate Apple Pay. This allows you to choose another payment method or a loyalty card.

To open your apple wallet (well, not really – it will actually invoke your chosen payment card) you need to double click the power button, not the side button (also referred to as "action button" on the system settings).

Side Button is the official Apple name for what you’re referring to as the power button. Action Button, on those iPhones that have it, is above the Volume up button. iPhone 16’s have a Camera button below the Side Button.

This Apple Support article will help.

Use the side button, Home button, and other controls on your iPhone - Apple Support

If I activate Apple Pay with a double-click of the side button, it will display your default card for Apple Pay. However, at the bottom of the screen area, you will see a collection of other cards in your Wallet, which for me, includes all of the other cards in my Wallet. I can access my supermarket rewards card, my airline mileage cards, my vehicle insurance cards, etc. I'm not sure why you are not seeing them. However, the other information you have been provided to access the Wallet app and access those cards is accurate. The only difference is that when tapping on the Wallet app, that does not activate the Apple Pay.