It doesn’t require an app. When you pay, you select your bank and it will redirect you to a page that the bank provides. My bank provides a QR-code I can scan with their banking app, but it also offers a log in form to pay.
So I guess it is based on what your bank is willing to provide.
This is based on my experience with ‘iDeal’ the predecessor of wero.
It is just a framework. OpenID en OIDC are probably used by those companies to federate user access.