But nothing like that happened here... in the slightest?

This was OS chrome integrating Facebook features. An API for Mobile OS UI to integrate Facebook features.

How else do you think the OS provides FB functionality... after you enter your credentials into the Settings screen?!

I think you need to read a bit more carefully. This was not just API calls, these were data sharing partnerships--i.e. the device companies had access and permission to collect, aggregate, and store customer data in one place, like Cambridge Analytica did. Example quotes (emphasis added by me):

> They said its partnerships were governed by contracts that strictly limited use of the data, including any stored on partners’ servers.

> Tests by The Times showed that the partners requested and received data in the same way other third parties did.

Facebook customer data was collected and stored on servers owned by the device manufacturers. This is not analogous to a browser interacting with the Facebook web application.

Did the OS keep the data or use it in any way beyond what the OS needed for display etc?