The root filesystem is a bit of a special case, as it is typically mounted by initramfs/initrd (in most major distributions) or the kernel (with no-initramfs kernel configurations, similar to how classic Unix systems did it) as part of the boot process, and cannot really be unmounted like any other filesystems.
But it is not wrong to say that "the root filesystem
/dev/sda1 is mounted to
/". I think some old Unix textbook said something like "the root filesystem is magically mounted to
/", as the details of that special mount process are tricky and essentially only relevant to kernel developers working on the kernel start-up tasks.
When initramfs is being used, the initramfs image is mounted as
/ as the kernel starts up. The root filesystem is then mounted at some temporary location and
/sbin/pivot_root tool (or the equivalent system call as part of some other distribution-specific tool) is used to switch the places of the initramfs and the real root filesystem with each other. Then the real init process is
exec()ed from the real root filesystem, to free the last remaining open file handle to the initramfs, and then the initramfs can be unmounted and the RAM allocated to it freed.
It might be possible to use the same process again to switch from one root filesystem to another while the kernel technically keeps running, but in practice this is not usually very useful: you'll need to stop essentially all processes anyway to get all the open files on the old root filesystem closed, so that the old root filesystem can be unmounted after pivoting it. And when you get to that point, you're going to be effectively so far into the shutdown procedures that it will probably be simpler to just reboot. As a bonus, by rebooting in this situation, you'll get a positive proof that the boot works with the new root filesystem.