Add windows desktop support to Bluetooth Serial (RFCOMM) plugin

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Hello,
I am a newbie in flutter.
I need to add windows desktop support to existing flutter_bluetooth_serial plugin, so our application can run on PC and windows based tablet.
I have several questions.

1. As far as I understood , the flutter_blueooth_serial was written as app and not as standard plugin.
   Was it done on purpose and if yes ,why?
2. Should I create the windows part of the plugin also as app? And if yes, how can I add the plugin code to it?
   If I understand correctly for android the plugin JAVA code was added to separate directory
   
   And was defined as package
   
   Maybe I'm missing something else.
3. Can the plugin code contain threads (for device discovery for example)?
4. If I understand correctly, I can write the plugin in one of the following ways:
   a) in C++
   b) as DLL in C# and afterwards load the dll and call to the dll APIs from standard plugin code.
   Am I right?
   Which way do you think is preferrable?
   Please help.
   Thanks a lot in advance