How do you 'write' this code? It seems all of the functions rely on other functions. How/Where do you start?

Just follow the method chain (they're called methods when a function is on an object). If you're looking at a method and you notice it relies on another method, go look for that other method and see what it does. Then go back to the one you were on and you'll have a better grasp on what that does. As you read through the code, you'll start to see other methods that rely on the ones you've already read about and your understanding of the codebase will continue to grow.

To start a project like this, you go with the bare essential methods to make it work. Then you add features and refactor from there. The reason he has so many methods relying on other methods is to keep his code DRY. There's no secret method to being able to write code like this. Just practice, read other people's code and try to understand what is really happening in the code in small chunks instead of trying to know what the whole program does right from the start.

I'm pretty new to OOP as well, and haven't taken the Treehouse course on it, but I'd suggest starting on paper. List out what you think the needed functions might be, and some of the variables/arrays you think you'll need. Start grouping them into objects; you can also start with the broader perspective of what classes you think you'll want, and then break them down into the methods they'll hold. Draw out some UML diagrams or something similar if that helps you visualize it; draw arrows, doodles, whatever helps is fair game.

One somewhat related piece of advice I got from a good source, is to test early and test often. Write just the simplest version of a function, test it, add a little tiny piece, test, and so forth. You can use a console.log or an alert as placeholder code to see your functions working (for instance, console.log("Function executed!") as the last line of code). You'll save time tracking down errors later, and it's also satisfying to see each little piece start to add up.

think of object as a way to store things.

An object can store properties, functions, and objects. Basically object is this big dictionary. That can store info and do things.

OOP javascript makes things so much easier and awesome.

part of the beauty of the JavaScript programming language is its lauded "non-blocking, asynchronous" nature. meaning, you can send out for a data source and the rest of your code keeps running, it doesn't just do a strange halt whilst waiting for the data to return and complete; and, you can set-up, to a certain degree, your vars and functions in any order you desire, so long as they are instantiated and called properly.

thus, in JavaScript, you can have a number of functions called where you haven't read their declartion to know what they do (yet), as they are elsewhere later in your code or another *.js file you've included it in the proper order of script tags in the HTML. this is unlike PHP or other "synchronous" languages, which require you to instantiate and use your vars, functions, objects, etc., in the order they will be used/called, or you get an error. this is a double-edged sword in JavaScript, as it means you, as the developer can throw vars & functions in your code wherever you like, within reason; however, even tho' it may compile & run, too much randomness can make your code unreadable to anyone else reading/using it.

this can also get a bit mind-blowing when you get into things like "recursion" or "recursive functions," where a JavaScript function actually calls itself within that function you are currently defining. pretty weird, huh? but it works. for a good example of this, see this JavaScript recursion example video, here.

just some of the odd things to keep in mind when using JavaScript.