Raspberry Pi, Intel Galileo, and Arduino

A viable and interactive small screen display that works with the Cambridge-UK Raspberry Pi, the Asus Tinkerboard, the Intel Galileo, and the Italian (designed and manufactured) Arduino. The components of a working Linux circuit board with SD-card, small screen display and keyboard. And a simple 'C' application scroll text program can be uploaded to each of the four circuit boards.
If the components are so simple and inexpensive then why is so hard to find working examples of assembled circuit boards? Adafruit does include any documentation within its boxes. The Chinese electronic components work within limited and special electronic compatibility conditions. So many of my purchased display devices are inert when plugged and tested.