[Enhancement] Singe Project Platform folders with multiple platforms

We have `Platforms/iOS` and `Platforms/MacCatalyst` for platform specific code, but what if you have code that is the same on both platforms? We should figure out a convention for something like `Platforms/iOS+MacCatalyst` for these cases.

Start by comparing the existing Platforms/iOS and Platforms/MacCatalyst folder conventions and how platform-specific code is organized. Determine whether a shared iOS and MacCatalyst folder convention is feasible; done means the convention is agreed and its expected use is clearly documented.

Written by the indexing model from the issue text.