Is this a just a regular Raspberry Pi in a nice case?
No, it appears to be using the Raspberry Pi 3 compute module. That's essentially a small circuit board with the core components of an RPi3 already designed and assembled. They then solder/plug this module into a larger board to add connectors and peripherals to it. So there's some custom design involved. https://www.raspberrypi.org/blog/compute-module-3-launch/
* It's either the Compute Module 3 or the older (cheaper, slower) Compute Module 1.
* The schematics of the base board in the RevPi Core are open source: https://revolution.kunbus.de/download/1803/
* Normally you wouldn't open the RevPi Core case and solder stuff to the base board, though you're given all the information you need if want to do so. The expected use is to just mount the RevPi Core on a top hat rail and extend it with I/O or gateway modules via the PiBridge connector.