Here is a brief flowchart, which shows how Parsify Desktop evaluates expressions:
As you can see, most of the work is done by plugins. They have direct access to expression, as well as the math parser (so that they can modify variables and functions).
That sort of system gives you the ability to extend the application to a great extent. However, please be extremely careful which plugins you install. Each plugin has access to a Node.js environment and, despite running in a sandboxed environment, can be bad for your privacy and data security.
On the next page, you will learn how to create a basic plugin.