Hi iio7,
... and welcome!
I may not be the most qualified person to answer you, having run Obarun only for two months. But I'll try nonetheless. Other persons will correct me if I'm wrong.
What are specifically are the "changes", "modifications", and "additions"?
Had you read a few lines further of the same page, there is part of the answer to your question:
Systemd replacement is made by Skarnet's S6 supervision suite as init and by Obarun's own 66 service management. S6 and 66 is the heart of Obarun, it is what makes it unique and special from all other linux systems.
Basically, Obarun is Arch WITHOUT systemd as an init-system and with s6 and 66 instead, to init and manage services.
Does Obarun have its own package repository?
Same thing, the homepage of the site goes:
Obarun's packages and source reside in the streamlined [obcore], [obextra], [obcommunity], [observice], and [obmultilib] repositories. The official [core], [extra], and [community] repositories from Arch Linux are retained to fit the needs of the competent Linux user. This means you get Arch-Linux, AUR, and more, without systemd of course.
Additional Tools written in script language are available to simplify/help tasks like installing software from AUR, building packages in a safe environment, installing a particular system, creating a live version of your system, managing and modifying services, and so on.
There are specific Obarun repos for a few packages (the Obarun tools). But the aim is that after having installed Obarun (and you can even do that graphically from the live ISO), you get an almost vanilla Arch that lets you use pacman and install what you want from the Arch repos and the AUR.
If you want further reading and details, you may want to go and read this:
https://wiki.obarun.org/doku.php?id=overview
https://wiki.obarun.org/doku.php?id=pacman
And here is a nice resource by fungalnet (lots to read!):
https://sysdfree.wordpress.com/?s=obarun
Regards