"How do I write my code so that each file has scope with the others."

Any decent 'C' course, tutorial, or book will tell you this - it's standard stuff; nothing specific to Keil.

For Keil specifics, you need to read the uVision Getting Started guide, and work through the example projects in it.
Then you can look at the many examples which are installed with Keil, and more on their website, and this site, and other vendor sites, and ...