While participating in a Zoom meeting, what you see depends on what's happening in the meeting along with your own settings.
When a participant shares their screen, your screen automatically shows that shared screen. While viewing, you'll see menu item toward the top of your screen named "View Options". Click that to reveal the options, where you can exit full screen mode if you wish.
In various modes, you'll also see an option in the upper right area of the screen that allows you to toggle between "Gallery" view (where you can see all of the attendees) and "Speaker" view (where you can only see the people who are speaking). Note that in Gallery view, if you see an arrow on the right or left sides of your screen, that means that there are too many attendees to see all in one screen; click those arrows to scroll through the pages.