Answer 1

These __ are macros pid_t is a typedef that has been defined

macro like:
__returns_twice
The returns_twice attribute tells the compiler that a function may return more than one time. The compiler will ensure that all registers are dead before calling such a function and will emit a warning about the variables that may be clobbered after the second return from the function. Examples of such functions are setjmp and vfork. The longjmp-like counterpart of such function, if any, might need to be marked with the noreturn attribute.

Those are a bit advanced for SoloLearn courses but here is a link:
https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Function-Attributes.html

Answer 2

idk🧐