Choose Apple menu > System Settings, click Accessibility in the sidebar, then click Zoom on the right. (You may need to scroll down.)
Make sure that “Use keyboard shortcuts to zoom.” is turned on.
You can then use:
Command+= =Zoom-In
In other words, press the Command key while simultaneously pressing the “=” key will zoom in
Command+- = Zoom-Out
In other words, press the Command key while simultaneously pressing the “-” key (to the left of the “=” key / top row right) will zoom out