What exactly does fork return?

Question

On success, the PID of the child process is returned in the parent’s thread of execution, and a 0 is returned in the child’s thread of e

Answer 1

fork() is invoked in the parent process. Then a child process is spawned. By the time the child process spawns, fork() has finished its execution.

At this point, fork() is ready to return, but it returns a different value depending on whether it's in the parent or child. In the child process, it returns 0, and in the parent process/thread, it returns the child's process ID.