AFAIK, the Smart Connector protocol used by the iPad Magic Keyboard and similar accessories is proprietary to Apple, and Apple has not publicly released detailed documentation about its inner workings.
Also, AFAIK, Apple does not provide APIs or frameworks in iOS or iPadOS that allow developers to interface directly with the Smart Connector. Instead, this connection operates as a low-level hardware interface between the accessory and iPad, similar to how Lightning and USB-C connections work.
As far as what other devices that can use this connector, I know of the following: