How could I describe the difference between a platform (hardware/software architecture) and a mobile platform (Android, iOS, ...)?
Or may it be considered to be equal/exactly the same?
If you are referring to the words platform and mobile platform in general, you can describe a mobile platform as category of platform (i.e. a mobile platform is a type of platform, specifically geared toward mobile devices and applications). If referring to (hardware/software architecture) to (Android, iOS...), you can say that the difference is that the hardware/software architecture is the technology being used to support the mobile platform. For example, I would being using hardware or a platform to run the mobile platform.
Overall, I believe that the difference is in the fact that a platform is a more general category of technology, while a mobile platform is a specific type of platform.