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When you connect one or more external displays to your Mac, you can choose what appears on each display.
You can use video mirroring to show your entire desktop on each display.
You can extend your desktop across two or more displays, and choose which display shows the Finder and application menu.
When you connect your Mac to a TV or use AirPlay, you can also choose to share only a window or an app.
If more than two displays are connected, you can specify that some use video mirroring and others use an extended desktop.
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On your Mac, choose Apple menu
> System Settings, then click Displays 
in the sidebar. (You may need to scroll down.)
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Click the name of your display, click the “Use as” pop-up menu, then choose an option.
Note: If you’re using a Mac laptop or iMac, you must have an external display connected to your Mac to see these options. If you’re using a device with no built-in display, such as a Mac mini or Mac Studio, you must have more than one display connected to see these options.
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To set up some displays for mirroring and others for an extended desktop, start by setting up all displays as an extended desktop. Then, press and hold the Option key, and drag one display icon onto another display icon to mirror those two displays.
Connect one or more external displays with your Mac
You can connect your Mac to one or more external displays. You may want to do this to make it easier to work in multiple apps, switch between the things you’re working on, or just have more screen space.
Before you begin
Before you can connect your Mac to a display, you need to determine a few things:
What kind of video ports your Mac has.
How many displays your Mac can support.
Whether or not you have the right cables.
With this information, you can connect your displays to your Mac.
Step 1: Identify the video ports on your Mac
Before you can connect displays, you need to know what type of video ports your Mac has. The ports you have determine what kind and how many external displays you can connect, and how you connect them.
Use the table below to determine what video ports your Mac has.
Note: There may be an identifying icon near each video port, depending on your Mac. If there isn’t an icon, you can still find out which ports you have. See the Apple Support article Identify the ports on your Mac.
What it looks like:
Step 2: See how many displays your Mac supports
Next, you need to determine whether your Mac supports the number of displays you want to connect. To do so, check the tech specs for your Mac: Choose Apple menu > System Settings, click Help in the menu bar and choose [your Mac’s name] Specifications, then scroll down to Display Support or Video Support (depending on your Mac).
Step 3: Make sure you have the right cables and adapters
If your displays come with cables that match the ports you want to use on your Mac, you can use those cables to connect the display to your Mac.
If your displays don’t have cables, obtain cables that fit the available ports on your Mac and displays.
If you have display cables, but their connectors don’t match the ports you want to use on your Mac, you may be able to use an adapter.
See the Apple Support article Adapters for the Thunderbolt or USB-C port on your Mac to identify what type of adapter you need, see what it looks like, and learn what it can be used for.
Step 4: Connect your displays to your Mac
Connect your displays to your Mac using the identified video ports, cables, and adapters (if needed).
After your displays are connected, you may want to adjust their settings. Choose Apple menu > System Settings, then click Displays in the sidebar to see your displays’ settings, including resolution, brightness, and color profile. You can also decide whether to extend or mirror your Mac desktop across your external displays.
If you have a third-party display, check the documentation that came with the display to get more information on the display’s video ports and cables, and to make sure you’re connecting the display according to the manufacturer’s guidelines.
If the external display isn’t working
If your Mac doesn’t recognize that a separate display is connected or if the display appears blurry, you can try to detect it in System Settings, or try one of the other troubleshooting tips below.
On your Mac, choose Apple menu > System Settings, then click Displays in the sidebar. (You may need to scroll down.)
Press and hold the Option key to make the Detect Displays button appear in place of the Night Shift button, then click Detect Displays.
If your Mac still doesn’t recognize the attached display, try any of the following:
Unplug the display from your Mac, then plug it back in.
Make sure your display is turned on.
Make sure the cable you’re using is securely connected to the correct port.
Make sure you’re using the correct cable to connect your display.
Make sure you’re using the correct adapter, if needed. See the Apple Support article Adapters for the Thunderbolt or USB-C port on your Mac.
Put your Mac to sleep and then wake it.
Make sure your Mac supports the type and number of connected displays. To do so, check the tech specs for your Mac: Choose Apple menu > System Settings, click Help in the menu bar and choose [your Mac’s name] Specifications, then scroll down to Display Support or Video Support (depending on your Mac).
If you’re using a third-party display, check the included documentation to ensure you’re connecting the display according to the manufacturer’s guidelines.