Sounds like it's a framebuffer, and yes the apps draw themselves.

Looking at the source code of the cursor app [0], we can see it draws the mouse cursor and skips the rest. The transparent console app [1] is doing something more complicated which I haven't tried to fully understand, but which definitely involves massaging pixels and even temporarily saves pixel data in ctx.store.b1 and ctx.store.b2 so it looks like some kind of double buffering.

  [0]: https://github.com/Ruddle/Fomos/blob/cba0460af59e63f46c7646f8a2f29d574ff0d722/app_cursor/src/main.rs#L75
  [1]: https://github.com/Ruddle/Fomos/blob/cba0460af59e63f46c7646f8a2f29d574ff0d722/app_console/src/main.rs#L384