The themes reside in /usr/share/plymouth/themes/ .
You set the default theme with:
# plymouth-set-default-plugin themename
# plymouth-set-default-theme themename
to make it occur at boot.
Plymouth works by using a series of still images to create animation for the display during the init process, so it loads along with the kernel. The scripting uses its own language which is said by one of the developers to be "not hard" to figure out, meaning that we mere mortals may likely have only a difficult time creating our own Plymouth themes.
Plymouth - ArchWIKI
Plymouth - Ubuntu