This was a specially made device that would connect via the link port and the barcode of these cards which would work as input to a small number of games that used this:
A very cool device that increased the number of buttons on the gameboy massively, of course only useful on carts that would interface with this device:
I found this really fascinating. In the video he says "we've added some really cool special electronics so that you can watch your game boy being played on a tv screen". I would love to know what electronics! Would it be difficult for someone today to build a gameboy to CRT converter? I couldn't find any schematics for the retail display.
You can see a ribbon cable going into the game boy itself, and a small electronics board - but I think it's just the volume controls. It's not at all clear how this device works!
This plugs into a famicom/NES and lets you play game boy games on a screen - it's basically a NES cartridge with a gameboy CPU on it! Another page on the wideboy here.
A similar type of thing called the wideboy64 exists, letting you play game boy color games on an N64!
These were never available to buy (because they were either used in retail store displays or just for developers) but the GameBoy Booster was sold which let's you play game boy games on an n64:
A really cool hack someone did is interpreting the game boy signals from the ribbon cable using a microcontroller (but I have read that an FPGA is much more suitable) and then converting it into data for an oscilloscope to display: