Apple - Is there a way to switch Mac OS boot partitions without going through System Preferences?

You can hold down the ⌥ Option key (also known as Alt) at startup, and that will show you all the available boot volumes.

If you want a boot manger which behaves similar to GRUB, the you could install rEFInd. This free boot manager is similar, because rEFInd can be configured to display a menu of operating system choices for a specified period of seconds, then automatically choose the last used if the period expires.