iMac zoom out of control

1 - Is the user using the Command + ( Plus ) and Command - ( Minus ) keyboard combinations is error ?

2 - Or possibly the Mouse option to Double click to Zoom In and Out

refer to image below

if I may add my two cents...

since your issue appears to have been "smart zoom", you may want to try and get used to how it works. ever since I've started using it, I have wondered how I ever lived without it. :)

command-option-8

turns off the zooming feature as quickly as it turns on.

Go to Apple menu -> System preferences -> Accessibility only if you need re-enabled.

Welcome and thank you too