This is how I achieve this.
-Click on Hot Corners and click on one of the corners. Once selected, pull down the menu next to it and choose Put Display To Sleep. Connect the external display to your MacBook and then move your mouse to the corner you selected above. It should turn off only the internal display.
If you have an Apple adapter and your external display isn't working, try the following:
If so, here's how you enable it:
By visiting > System Preferences > Display in your Mac's menu bar, you can visit the Colors section and choose a color profile that looks the best to you.
Disable your Mac's display with Hot Corners Open System Preferences and click on Desktop & Screen Saver. Select the Screen Saver tab, then you should see a button marked Hot Corners… under the main pane. Click this and you'll see a mini window appear with drop-down menus for each corner.
2020 MacBook Air Supports Up to 6K External Display, Including Pro Display XDR. Apple today announced that its new 2020 MacBook Air supports up to a 6K external display for the first time ever, including the Pro Display XDR.
Go to System Preferences > Display and try selecting a different resolution. If that still doesn't work, try restarting your Mac in safe mode and then reset the display resolution to defaults. Another option is to reset the Mac's NVRAM and SMC.
Go to System Preferences > Displays, click Scaled and hold down the Option key to show and select the Detect Displays button. Go to System Preferences > Displays and check the resolution and brightness settings. Also, check the display cable connections, and update adapter software if applicable.
Calibrate your Mac display
No. Using an external monitor or leaving the computer plugged in will not damage the computer or the battery. Leaving the computer plugged in reduces the rate of accumulation of battery cycles.
If you have a 27-inch iMac with a Mini DisplayPort , any other Mac with a Mini DisplayPort or Thunderbolt port can use it as a display. Make sure you have a Mini DisplayPort to Mini DisplayPort cable. If the primary Mac has a Thunderbolt port, it can play its audio and video on the iMac.
Connect an HDMI display or HDTV: Plug a cable from your HDMI display or HDTV directly into the HDMI port on your 14-inch or 16-inch MacBook Pro. Use a USB-C Digital AV Multiport Adapter to connect the HDMI display or HDTV to a Thunderbolt / USB 4 port on your 13-inch MacBook Pro13-inch MacBook ProApple 60W MagSafe 2 Power Adapter (MacBook Pro with 13-inch Retina display).