Where one can find the built-in onclick method of a button within a form tag?

Question

I'm trying to understand a little of how the following html code snippet works under the hood:

<form action="/some_action" method="POST">
    <input name="name">
    <input name="email">
    <input name="password">
    <button> Sign Up</button>
</form>

I was expecting to find that button has some built-in onclick attribute that describes what it should do when it gets clicked.

I've tried inspecting the button and its respective form element with document.getElementByTagName() method and printing them to the browser's console with console.dir(), but hopelessly so far.

When the submit button is clicked, the GET request is triggered, but the button attributes and all other methods are empty strings when I inspect the document. It doesn't make sense to me. How the button does its job if it doesn't contain the information to do it?

Hope someone will help me so I can get on with my life : ]