To be even more precise: filters
In OBS, you can add an "image mask" filter. This allows you to import a shape (a circle, a rectangle, etc.) to crop exactly what you want to show.
Concrete example
Want to show a single exercise from a PDF?
Just capture the PDF window.
Crop around the exercise.
Result: students won't see your menu bar or notifications.
In short
To show a single area in OBS: use Screenshot or Window Capture, then crop. It's simple and avoids distracting students.