Note: this is currently only a "placeholder" for a standards proposal along the lines of the following picture:
This is essentially an Operating System, albeit a dedicated one.
The goal is also to eventually make banks' own services like online banking, use the same API.
Not all, loose coupling is firmly established since decades back. More recently, Micro Services and messaging systems like Kafka, proved that networked interfaces nowadays represent a viable alternative to native mode APIs.
The only "novelty" is standardizing a set of core methods and an associated security solution.
Unlike most current Open Banking API concepts, Open Banking 2.0 permits testing application-specific APIs using public sandboxes. No need to build specific test banks!