If you have a Mac, and it has multiple hard disks or partitions containing operating systems, including Windows installations in BootCamp partitions, you can set any of them as the default 'Startup Disk'
1. Open the System Preferences app from the Apple Menu, or from the Dock.
2. In System Preferences, click the 'Startup Disk' preference pane.
3. In the Startup Disk preference pane, click the lock in the lower left to enable changes, and then click the volume that you want to make the 'default' start-up volume. In the example, the Windows Bootcamp volume is selected.
4. Click the 'Restart' button to start up from the selected volume, and thereby the selected operating system.
Remember, this setting changes the default operating system, so if the machine is restarted, and no action is taken to select another startup volume, it will choose the one selected here. To change this setting from Windows on a BootCamp-equipped Mac, see Reboot to macOS from BootCamp Windows.