An extension tube doesn’t crop the image in the digital sense. It changes the lens-to-sensor distance, which changes where the lens projects its image.

A camera lens projects an image circle onto the sensor, similar to a projector casting an image onto a screen. When you move the lens farther from the sensor with an extension tube, that projected image becomes larger at the sensor plane. The sensor stays the same size, so it records a smaller central portion of that enlarged image circle. That means the field of view becomes narrower and magnification increases.

So the result can look crop-like, but it is caused optically, not by trimming pixels after capture.

This is also why extension tubes let you focus closer: increasing extension is like racking the lens farther forward, as happens when focusing away from infinity toward near subjects. The tradeoff is reduced working distance, shallower depth of field, and typically some light loss. In short: extension tubes enlarge the projected image on the sensor plane rather than electronically cropping it.