Pass2U Wallet works great, but like many apps, it really should just be a feature to begin with.
The weird thing is that this was a feature when Wallet was first introduced. You could create a URL that would add a pass to a wallet. There were web sites that helped out with it. I still have some of the passes I created this way a decade or more ago.
Pass2U is neat because you can make somewhat arbitrary "Cards" with it. Cards with nice layouts that include QR code, images, titles, text, whatever. I have a card for my drone license, insurance, medical, and a few others. Super handy. It would be neat if they allowed that level of customization in the wallet app itself, but this is Apple.
The weird thing is that this was a feature when Wallet was first introduced. You could create a URL that would add a pass to a wallet. There were web sites that helped out with it. I still have some of the passes I created this way a decade or more ago.