Need to understand why few mobiles have multiple frame buffers

If I understand your post correctly, you're either referring to multiple render targets or some evidence of multi-pass rendering.

On some OpenGL applications, special effects are achieved by rendering part of a scene onto a buffer, then using that as a texture in another rendering pass. In OpenGL ES 2 this is slightly more work than in OpenGL ES 3. The latter supports multiple render TARGETS, which differs someone in performance and feature, but is similar to the "2" method of using the output of a rendering as a texture in another rendering pass.

Reflections and shadows are typical examples.

You can only know what is which by the conventions of a particular application - there is no particular standard, as from what I infer from your post you are seeing these from a kind of reverse engineered perspective.