Hey there ATOMEK3000,
Thanks for reaching out to Apple Support Communities. We understand that you're wondering how to select the best resolution for an external display. We're happy to provide some assistance.
Check out this helpful resource under the "If you're also using an external display" section:
Using a Retina display
"If you're using an external display to mirror your built-in display, your Mac optimizes for whichever display is selected in the ”Optimize for” pop-up menu. Allow your Mac to choose the best resolution for that display, or select Scaled and choose a different resolution."
We hope this helps.
Have a great day!