GetPlatformBackendClass is a protected class method that returns the TCustomBackend derived class to use when the constructor doesn't explicitly specify one.
By default GetPlatformBackendClass returns TMemoryBackend, but derived classes can can override GetPlatformBackendClass to return something else. For example, TBitmap32, overrides GetPlatformBackendClass to return a suitable platform specific backend class, such as TGDIBackend on Delphi/Windows or TLCLBackend on LCL/FPC.