A lot of modders/tinkers like to take game consoles and turn them into all kind of things. For example, the famous benheck has done this to countless game consoles like the Xbox360 & PS3. 
Modder GmanModz has also done this to a bunch of systems and now he’s turned a Gameboy into a functioning Wii. All the games are running off the SD card as there’s no way you could squeeze the DVD drive into Gameboy. Some of the parts he used came from various systems, like the circle pads are from the 3DS, the D-pad is from the DS lite, and the buttons are from a Gamecube controller. GmanModz has also equipped it with a headphone jack and portable battery.
If you like to know how GmanModz was able to trim the Wii and fit it inside of a Gameboy I suggest you take a look at this guide here. The definitive wii trimming