SDL programming question

Maybe hes doing it in x86 so that it would be backwards compatable with a 32 bit operating system?

This is the most likely reason. It saves the trouble of building a both a 32-bit and 64-bit version. Another possible reason to compile as x86 instead of x64 is that you may be using some libraries that you only have in 32-bit form.

You know which .lib files to use because the documentation of the library you're using tells you. Of if that fails, you guess and see if you get linker errors.

A linker takes multiple object files and static libraries and combines them into a single file. In conventional C or C++, each .c or .cpp file creates an object file when compiled.