Does it make any actual differences whether I set FlaxEngine(C#) or Flax(C++) as the startup project?
Yes, it makes a difference for the debugging experience. The FlaxEngine C# can only debug the C# stuff, the Flax C++ can only debug the C++ stuff.
Does it make any actual differences whether I set FlaxEngine(C#) or Flax(C++) as the startup project?
Yes, it makes a difference for the debugging experience. The FlaxEngine C# can only debug the C# stuff, the Flax C++ can only debug the C++ stuff.