Windows is the one I know best. The current native accessibility API for Windows is called UI Automation. It's not tied to any high-level UI framework, though a UIA tree needs to be associated with a window handle (HWND).
Mac and iOS have Objective-C accessibility APIs as part of their respective native UI frameworks (AppKit and UIKit). I have cursory knowledge of those APIs from a previous job but never implemented either of them from scratch. Android likewise has a Java-based accessibility API as part of its framework. Yes, this means that non-Java-based toolkits have to do JNI bridging to implement accessibility on Android. sigh I've never done this myself either.
The only desktop environment for the free Unixes that has full-featured accessibility support, particularly for blind people, is GNOME, which has a D-Bus-based accessibility API called AT-SPI. GTK implements this API through a module called ATK. Qt also implements it. BTW, the Orca screen reader for GNOME was originally developed by a friend of mine.
Probably the best place to find an implementation of all of these APIs is one of the open-source web browser engines. Note though that on Windows, these engines implement a legacy accessibility API called MSAA (Microsoft Active Accessibility) and an unofficial extension of that API called IAccessible2. Chromium has a work-in-progress native UI Automation implementation, largely developed by the Microsoft Edge team.
Disclosure: I work at Microsoft, on the team that develops UI Automation and the Narrator screen reader.