Pre-Built versus Custom functions

I am new to Unity and might not know the right terminology.

What is the difference between a pre-built function like Update, versus when I make a function: function FunctionName() {};

I can call custom functions, but what if I wanted to call the Update function within another one. Can I do that? If not, is there a way to simulate an update function, or am I going to need to create a separate script and work around it?

Am I missing something crucial here?

Yes, I think you are missing something crucial. The functions that Unity defines (Awake(), Start(), Update() and many many more) are called by Unity to allow your code to do something. Update(), for examples is Unity giving you a chance to update your objects, and process things, before each frame is rendered. There’s no real reason I can think of for one of your functions to need to call Update(). And it would be wrong to do that. And I cannot think of why you’d want to simulate an update.