I need a package that is only available in the staging repo. I cannot wait till it becomes available in the stable repo. So, my question is if I install the package from the staging repo, what will happen when the package becomes available from the stable repo? And from where will the package be updated? From staging or stable repo?
stable-staging is not a repository but a branch, that serves to check how the next stable update behaves with the current snapshot.
Since it is a branch, it is recommended to make a full update after switching to that branch, in order to avoid getting an unaligned system.
You can then switch back to stable, though you will then be ahead of it until that branch finally gets updated. https://wiki.manjaro.org/index.php/Switching_Branches