This website is built on the SvelteKit framework. Svelte modules look a lot like ordinary HTML pages, with standard HTML tags and some additional Svelte features.
The modules in this presentation are small and isolated from other modules, making them easy to understand and maintain. It's often good coding practice to prevent bugs by avoiding mutation, side effects, and global variable, and to impose type constraints. But when modules are small and isolated, these precautions unnecessarily add lines of code and make things more complex and complicated than they need to be.
"Monad" is rigorously defined in Category Theory, but not in this presentation. Here, a monad is a closure that returns a function that facilitates the composition of values, including anything from asychronous functions to primitive values. Sometimes, like Haskell monads, they shield a series of computations from possible interactions with other code. Usually, they are a convenient means of composing functions.
"Functional programming is an approach to programming based on function calls as the primary programming construct." - Greg Michaelson on Page 3 of An Introduction to Functional Programming Through Lambda Calculus. That's about all I mean by "functional programming", adding only that it is sometimes advantageous to rely on small, reusable functions as arguments for the specialized functions that accomplish the goals of a module.