Hi, in Settings > Wallet & Apple Pay > cards listed were once added to your Apple Wallet app on your iPhone.
In the Wallet app are cards that have been authenticated by the issuing bank. What appears in the Wallet app are cards that can be used for payments using Apple Pay and for transfers involving Apple Cash. These cards have no relationship to the Keychain.
Cards saved for Autofill are synced across devices using end to end encryption with Keychain. When Keychain is enabled for iCloud the information is also stored in iCloud with encryption. Cards can be in Autofill without being in the Wallet app and vice versa.
Cards in your Apple Wallet app are used exclusively with Apple Pay and Apple Cash. They use tokenization which replaces the actual card details with encrypted payment tokens. When Apple Pay is used either online or in store, the merchant never has the actual card details, but a one time use encrypted payment token for the bank. This is a key distinction.
Autofill provides the merchant with actual card details and they are at risk if the merchant is hacked by scammers.