How Atmel Studio 7 compile Arduino code?
It is almost always the case that a high-level environment like an IDE provides an interface to an existing tool-chain rather than being truly an all-in-one mega-application, which would a maintenance nightmare. Atmel Studio 7 is no exception, and according to Atmel, uses and includes (quite likely by downloading it during the install process) the GCC tool-chain.


  Why Atmel Studio asked about Arduino IDE path when I try to import Arduino sketch?


Probably for the Arduino libraries, which are contained within the Arduino IDE executable.