How to open a 'magnifying glass' on macbook pro. OSX 10.12.6

Open System Preferences> Accessibility> Zoom. These are the options for the Zoom tool,

as you have the box popping up you have set Zoom Style to Picture in Picture.

You have probably activated it by pressing the Alt/Option (⌥) and Command (⌘) and 8 keys

or the ⌥⌘= keys to make the Zoom box disappear press the ⌥⌘- keys.

Enable Zoom in OS X

Select Apple menu > System Preferences, click to open the Accessibility panel, and then click Zoom in the panel on the left.

Enable either of the following options:

To zoom with keyboard shortcuts, select "Use keyboard shortcuts to zoom."

To zoom with your mouse or trackpad, select "Use scroll gesture with modifier keys to zoom."

Decide where the magnified content appears with the Zoom Style pop-up menu. You can display the magnified content in a window near the mouse cursor by selecting the Picture-in-picture option. You also can display it on the entire screen by selecting Fullscreen. I prefer the first PIP option as that allows you to zoom in on a small section while still viewing the entire screen.

To set options for zoom or the zoom window, click More Options as described on Apple's support site to change magnification, window position and other settings.

Alternatively, you also can turn on zooming quickly by pressing Command (⌘)-Option (⌥)-F5. This opens an accessibility panel that allows you to enable or disable the Zoom options.

Enable Zoom in OS X

Select Apple menu > System Preferences, click to open the Accessibility panel, and then click Zoom in the panel on the left.

Enable either of the following options:

To zoom with keyboard shortcuts, select "Use keyboard shortcuts to zoom."

To zoom with your mouse or trackpad, select "Use scroll gesture with modifier keys to zoom."

Decide where the magnified content appears with the Zoom Style pop-up menu. You can display the magnified content in a window near the mouse cursor by selecting the Picture-in-picture option. You also can display it on the entire screen by selecting Fullscreen. I prefer the first PIP option as that allows you to zoom in on a small section while still viewing the entire screen.

To set options for zoom or the zoom window, click More Options as described on Apple's support site to change magnification, window position and other settings.

Alternatively, you also can turn on zooming quickly by pressing Command (⌘)-Option (⌥)-F5. This opens an accessibility panel that allows you to enable or disable the Zoom options.

https://powerator.co/

In Preview, you press the tilde/backtick key to open the magnifier:

which looks like the following, and can dismiss it by pressing the esc key above it.

Literally a magnify glass is controlled by system preferences > accesibility > Zoom

or the keys John Galt suggested.

"made the whole screen bigger" in certain Apps is controlled with Command + and Command -

Yes I know you didn't open the System Preferences> Accessibility> Zoom options, this was to show you how the options for Zoom have been set up. Once they are set up you just press the keys as described and you will get the Zoom box/ magnifier, you do not need to have System Preferences> Accessibility open to use them.

If you never set your Zoom options up when you first got your mac then they would have been set to the defaults.

You can now choose to keep them on, or switch them off.

The mac is full of keyboard shortcuts you may not know about and by clicking on a combination of keys

by mistake may invoke one of these shortcuts just as you have found inadvertently with the Zoom keys.

You paid a lot for your mac, you should really learn how to use it.

Have a look at System Preferences> Keyboard> Shortcuts go through the list of options on the left of the panel and look at the shortcuts on the right hand panel, you can switch them on or off or change them to

ones you are more likely to remember.

There is the magnifier which is the rectangular view, and there is a Loupe Annotation (control+command+L) which is circular. The first is dismissed by pressing the backtick key again (or esc), and the Loupe Annotation is removed by clicking on it for selection, and then pressing the delete (backspace) key.

The magnifier is intended for interactive use by you, and the Loupe is something that you position over text, set, and save the PDF — so that text under the Loupe remains magnified upon reopening the PDF.

If you're seeing a rectangle surround the cursor, that's caused by one of the many options available in Make it easier to see what’s on the screen of your Mac.

Refer to Use zoom.

⌥ is the notation for OPTION-KEY

If you don't want to fuss with keyboard shortcuts, use

System preferences > Accessibility > Zoom

to access the controls and turn Zoom feature and its shortcuts OFF.