It uses the internal APIs mobile banks use afiak. For what it's worth the open API laws aren't in power yet in Europe and tbh I imagine nearly all banks will fail to implement by the deadline. Or if they do, the APIs will be fairly crippled.
So does Europe. Open Banking Ltd in the UK is attempting to develop these standards, but they are working with tens of stakeholders across impossible deadlines - which means they cut scope and quality. And since banks in the UK are sponsoring the show, it is beyond certain that no disruptive moves will be taken, and interests of the incumbents will remain preserved.