Yes, the malware described in the CCC's document is a Windows DLL file.
Yes. And why? Probably because the majority of the personal computers of German citizens use Windows as their operating system.
So, if you are in the 10% of Mac users, you are of no interest to the authorities? Quite pragmatic, I suppose.
It targets Win32.
This is actually important to note: the software depends on an unsigned 32 bit kernel module. If it were 64 bit, it would have to be signed to function, so this particular piece of malware will only work on Windows 32.
That said, there might be Bundestrojaners for 64-bit Windows. Or even entirely different operating systems.