This is a custom Arch-based configuration that works by hijacking certain core packages inside the Arch Linux Package Repository. It adds custom dependencies to the core packages like base and filesystem.

Because it is fun and I don't want to spend time configuring each piece of the system one by one, over and over again since I reinstall my system occasionally.

Here's a diff of how you're going to do that using /etc/pacman.conf

...

+ [kettei]
+ Server = https://ketteilinux.github.io/kettei-depo/base

[core]
Include = /etc/pacman.d/mirrorlist
...

and for copy-paste, here's the raw version:

[kettei]
Server = https://ketteilinux.github.io/kettei-depo/base

After doing this, you should install a fresh system using pacstrap since updating your current system might result in unforeseen consequences. Make sure that your boot mount points to /usr/boot/

I recommend you to follow sainnhe's instructions on their blog. Here's the blog-post btw