When you start up a Mac holding down the Option key, the boot volume menu appears and displays all available bootable volumes. When you select a drive by clicking on it or selecting it with the arrow keys on your keyboard, it’ll boot from that drive until the next reboot. At the next reboot, unless you hold down the Option key again, the Mac will boot from whatever drive was set in the Startup Disk preference pane to be the default boot drive.
However, there’s also a way to change the default boot volume from the boot volume menu. In this case, once you got to the boot volume menu, you can let go of the Option key, then hold down the Control key. At that point, you should see the upward arrow icon that points at the currently selected drive turn into a circular arrow.
Once you select that drive and boot from it, it will now also be set as the Mac’s default boot drive.