How are certain #define(s) setup to compile into intermediate folder?

Curious how this mechanism works where certain #define(s) for compile are generated into the intermediate folder like so?

C:\UnrealEngine-master\UnrealEngine-master\Engine\Intermediate\Build\Win64\UE4Editor\Development\Sockets\Definitions.Sockets.h

and then contains #define(s) like so:

#define SOCKETS_PACKAGE 1
#define SOCKETS_API DLLEXPORT

How are those setup so they go in to the intermediate folders during compile? Doing a search through code didn’t reveal that to me.