Manjaro is indeed a rolling-release distribution, which means that instead of installing fixed-point releases every six months to a year — as with distributions like Ubuntu, Mint, Debian, RedHat, et al — you install Manjaro only once and you keep it updated. The updates will always put you on the latest release.
pacman-mirrors --status only gives you the status of the mirrors in your saved mirror list. Those mirrors are the same between Stable, Testing and Unstable. They are only the servers containing the packages.
Switching between branches is also always a conscious decision — it does not happen haphazardly. After installing Manjaro, you are always on the Stable branch, and if you want to switch to Testing or Unstable, then you need to explicitly do this via pacman-mirrors. Furthermore, switching branches requires superuser privileges, so you cannot just accidentally do it without realizing what you’ve done.