Where are stored C "object" and "executable" files in Xcode?

1. The files are stored in the project directory; in Xcode, find the file you'd like to see where is, right-click it, and choose "Show in Finder".
2. Nope. You'll have to recompile for other platforms, and possibly change code to be more portable. (That last, of course, depends on what libraries you use.)
3. Probably. Xcode may set some compiler flags you wouldn't in the terminal. Look in your project settings for these.
4. I would expect so, but I do not know how.
5. Clang from the LLVM project. Apple used to use gcc.

This is from my Yosemite (10.10.4) Mac with Xcode 6.4:

$ clang --version
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.4.0
Thread model: posix

Amusingly, gcc also is actually clang:

$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.4.0
Thread model: posix

6. If you want to develop native Mac or iOS apps, you pretty much have to use Xcode. Java apps are not native, and you cannot write Java code in Xcode. (One used to be able to write Java in Xcode back before OS X 10.4, before the Cocoa-Java bridge was deprecated.)