Inside an Official GameBoy Dev Cartridge

A good reference are the Microchip ANs (Application Notes). They'll tell you all you need to know about a specific IC or technology. They even have an AN about electric motors.

You could try grabbing a digital electronics textbook such as this one (link below). I haven't read this one specifically.

http://www.amazon.com/Digital-Electronics-Practical-Approach...

I took some EE courses as part of a CS degree. Years later, I found that reading a textbook was a good way to re-learn it.

It's not too late, you can take free courses online about digital circuitry. You can also take any electronic device you have taken apart and just google the part numbers on the IC's, it should lead you to datasheets that explain what each one does and many of them include a reference design so you can rip it off the board and add the components mentioned in the datasheet to make your own gadget! You should also look into microcontrollers, they are very fun and act as a gateway between software and digital electronics, so if you can program you'll enjoy this. There are many development boards on the market and a lot of free software that you can use to program them.

From the site of the company that makes most of Nintendo's dev equipment, there was a gang writer that the cartridge plugged into.

https://intsys.co.jp/english/tools/index.html

There was a cartridge programmer that you plugged the cart into.