How to clone startup disk?
Cloning isn’t easy in my experience since you need to first learn a bit about specifics and how to select a proper clone tool tool and understand any filesystem differences between your exact versions of OS.
Why not install a fresh OS on the SSD and then when it first starts migrate everything from the old OS?
Apple tools handle this for you including all the version difference if you choose a different OS on the new disk. And with two disks, you can thoroughly test the new drive before you install it or wipe the internal drive.