What does Set as Main Display change?

Set as Main Display makes the selected screen the main display in the macOS desktop layout. It does not start mirroring, open a stream or make a virtual screen appear on a physical monitor.

Choose App Menu > [display] > Set as Main Display. The selected display becomes the layout’s origin, and the other displays retain their relative arrangement. macOS manages the resulting placement of windows and Spaces. Some apps also use the main display when deciding where to open windows.

A virtual screen remains a separate desktop in memory even when it is main. To see its contents on a physical display, configure mirroring, streaming or Picture in Picture. See virtual desktops and viewing methods. Associating a virtual screen with a monitor is not, by itself, a command to show its image there.

Changing the main display can move windows or Spaces, so do not rely on it as a predictable switch between several independent presentation desktops. If you want to choose which virtual desktop is visible, change the source being mirrored or streamed instead. See switching between virtual desktops. Check the visible source and the screen-sharing app’s selected display before presenting sensitive material.

For a keyboard action or a multi-step setup, add BetterDisplay’s Set Screen as Main action in macOS Shortcuts and choose the display explicitly. If the same Shortcut also connects that display, put the connection action first. Main-display selection and display connection state are separate actions. See Set Screen as Main and connection presets.