Mount points are specific to one install - for example, you can mount your Manjaro root partition as /mnt/manjaro on Fedora. From every distro’s perspective, the partition it is installed on is /.
You seem to be mixing up the locations of partitions and mount points - a partition is somewhere on a disk and a mount point is basically a sign that points to it, and every distro can have different signs that point to the same thing.