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.