It can often be useful to zoom in on parts of the screen to make small text bigger, fill the screen with an image or movie, etc. To make this easy, use the zoom feature in the accessibility options...
Go to the Apple menu, then "System Settings". Click "Accessibility". Click "Zoom". Switch on "Use scroll gesture with modifier key to zoom". For "Modifier key for scroll gesture" select control. Close "System Settings".
To use this feature
Move the pointer to the center of the part of the screen you want to magnify. Hold down the control key. Use a two finger push up gesture on the trackpad to zoom in. Use a two finger push down gesture on the trackpad to zoom out. For a mouse, use the scroll wheel instead of the gesture.
Note: these instructions are for Sonoma. Other systems could be slightly different. For example, in earlier system, use System Preferences instead of System Settings.