Yum and GRUB

[XP_2600]

I have a question, when i run yum update, its update the centos Linux kernel bringing the newest available one, and after rebooting, we find the new kernel listed in grub boot menu, technically which tool written it? , is it part of kernel update package which yum ran? what if i downloaded a newer kernel not from yum, can i add it manually to grub menu ? thanks.