How to use TV for a monitor
What is the Command keys to use a TV Monitor for MacBook Pro?
MacBook Pro 13", macOS 10.14
You may be thinking of the command to toggle "Mirror Displays",
That is F1 (which may require the fn key on some Macs).
There is no requirement to disable the built-in display when also using an external display, they can be used independently in several different ways.