What's the difference in internal construction between a photocoupler and a digital isolator?

A photocoupler is a device in which a LED and a light-detector element are built in the same package, electrically insulated by resin, where signals are switched ON/OFF by the LED. Meanwhile, a digital isolator is a device in which a modulation chip with insulating film and a demodulation chip are contained in the same package, and signals are transmitted between the chips by a magnetic or electric field.

For Standard Digital Isolator products, please refer to the following links.