The quick & dirty question I have is... What app/script actually loads the Grub 2 menu system? And where is it located?

The longer question is I understand the core.img is loaded onto the first 63 sectors (stage 1.5) and after it loads I starts the stage 2 Grub bootloader which loads the Menu system. I've searched to try & figure this out but all I keep seeing are references to an ambiguous "Grub 2 bootloader"... What is the exact process of transferring the boot process between stage 1.5 and stage 2?

Can someone PLEASE help me with the specifics... at this point finding the answer has kicked in my OCD & it's driving me crazy.... Thanks!!!