Manjaro inherits most of their package bases from Arch (Stable), plus a few from the AUR, along with their own software tools, kernels, and utilities. It works like this, in general…
- Manjaro Unstable repository pulls from Arch Stable repository.
- Manjaro Testing repository inherits from Manjaro Unstable.
- Manjaro Stable repository inherits from Manjaro Testing.
As you can see, a lot of work takes place before what begins as vanilla Arch packaging ends up as a polished Manjaro release. That is why Manjaro is not Arch Linux–it is much more so. It stands on its own very well, indeed.
That’s the nutshell version of the topic question.