How am I supposed to download the wallet app?

There is not a Wallet app on the iPad. You have access to Wallet and Apple Pay via Settings. You should see the listing "Wallet & Apple Pay" in Settings on the iPad. It allows you to enter Payment cards and transaction information, but it does not show a separate app like you see on the iPhone.

Your profile shows that you are using an iPad Pro with iPadOS 17. I am assuming therefore assuming that your question is related to the iPad.

There is no Wallet app for the iPad. There is, however, the Wallet app for the iPhone that is included and comes pre-installed with the iOS.

The way this works is that you would add your cards/passes to the Wallet app on your iPhone. If you have an iPad that is signed on to the same Apple ID as your iPhone, then those cards (not passes) will appear on the iPad in Settings ▶️ Wallet & Apple Pay.

Axel F.