CALC 1 uses JavaScript to allow users to manipulate a global "c1" object that contains the calculator data. The c1 object also has over 120 functions that can be used in addition to built in JavaScript objects like Math, Array, Date and Number to name a few.
You can see many examples of how it is used in CALC 1 by looking at the scripts of the keys with the Key Editor and the function with the Function Calculator Editor.
If you are not familiar with JavaScript but would like to customize CALC 1 there are many sources to help you learn the language. One of the reasons JavaScript was chosen for CALC 1 is that it has been a top language for many years. There are a lot of sources to help learn it, and a lot of tools you can use to develop great solutions. It will be worth the time invested and something that can be added to a professional resume.
JavaScript supports much of the structured programming syntax from C (e.g., if statements, while loops, switch statements, do while loops, etc.). So there are not only existing functions and code written in JavaScript that can be used in CALC 1, but also functions written in C or the many other "C based" languages that can be translated to JavaScript.
See the CALC 1 c1 Object Reference for details about the c1 object.