I created a rectangle in Illustrator, placed an image on top of it, then copied the rectangle, placed the copy on top of the image and clipped the image with the copy. So the result was a clipped image on top of the first rectangle. The strange issue I don't understand is why when I drag a selection box over the clipped image, the rectangle in the back gets selected as well. When I just click on the clipped image, only the image is selected, which is what I expected. So it seems Illustrator selects different things depending on the selection method (single clicking vs drawing a selection box). In any case it seems strange that the rectangle in the back gets selected as it is completely covered by the clipped image on top of it. It's not a bug as I've seen this behaviour in a tutorial, but still I don't understand why the selection tool works this way.
It is working as expected. Box selection in most object oriented drawing applications selects things in the box. Thats the purpose of box selection to select many things quickly.
This is not at all a unusual feature:
it works like this in many CAD applications. Although they usually have an Autocad style extension where selecting left to right selects as all things that touches the box and right to left selects only things fully contained in the selection. Examples include but not limitted to: Autocad, Rhino, Solidworks, Creo, KiCad.
Vector drawing and desktop publishing applications.
Examples include but not limitted to: Most of adobe suite, corel draw, inkscape, affinity designer.
3D animation software usually do this aswell.
Application examples including but not limitted to: Maya, 3DS max, blender.
There are more applications like this in simulation tools, graph drawing tools etc.
It is thus a common feature indeed. Which is slightly unusual as usually many of these applications work under the premis if they didnt invent it then its not there.
Usually they also select things that are behind what you can see. Because most of the time you operate on the data thats there not just what you can see. See the process of doing happens in the brain so operators are usually expected to know what they or others have construcred by looking at the gui.
Though animation software especially later recognized that you may want to toggle between what you see and dont see. But then many applications have all sorts of selection filtering options.