Running macOS on an external SSD require sufficient USB port speed. 2012+ all have USB-3 speed USB ports. 2017?+ all have even faster USB-C ports
Here is a dual USB-3 and USB-C port 2.5" drive enclosure ( link )
See here ( link )
If getting an SSD pre-installed with macOS, then the macOS needs to match the latest macOS that was running on the Mac. Can not just boot a newer macOS on a Mac without going through the macOS installation process. This is because macOS installation will often install firmware onto the logic board. Installed macOS depends on this firmware. So if running a macOS on a Mac without the necessary installed firmware, there will be problems.
For example, if your Mac is running Catalina prior to the upgrade. Then a pre-installed macOS external SSD drive should have Catalina installed and not > Catalina macOS.
Power on Mac
Press and hold option key on Apple keyboard
Wait until see yellow drive icon (Yellow means external) Press right arrow key until yellow drive is selected and hit return
This will boot the macOS from external SSD
Set the default boot drive to this external SSD in System Preferences so don't need to do this step in the future. But if external SSD is unplugged and changes to different port, then probably need to set the defaults again.
This has additional complexity beyond most novices. Here is a video for reference ( link )
This is basically same process as an internal drive. Novice should seek an expert's help with 2 more complex details.