how do I get my external monitor to display what's on my macbook pro?

ckfromedmonton wrote:

how do I get my external monitor to display what's on my macbook pro?

When you select the "Mirror Displays," your Mac chooses a resolution that both displays appear capable of showing completely.

It switches both displays to exactly the same display generator Hardware, and they are perfectly in sync.

Use external monitors with your Mac - Apple Support

https://support.apple.com/en-us/HT202351

>System Preferences>Display

Get help with video issues on external displays connected to your Mac ..

https://support.apple.com/en-us/HT201177

and the graphic that goes with that:

The two blue Icons that represent the screen area of each screen will be nearly superimposed.