How do I edit the functionality of Add to Cart button?

Our site will sell flooring so I whipped up a conversion form that takes square footage input from the user and converts it to the number of boxes of flooring the customer needs. I’m trying to track down the scripting for the Add to Cart button so I can change where the button pulls the “quantity” to send to the cart. Not having much luck looking through the code and the Googling I’ve done yields 37,000 articles about changing the color and font of the button.

Using Dawn theme. Please advise.

Here’s the preview link for the store: https://9xtwuuaf92p0z5i0-76305367361.shopifypreview.com

Scroll down below the hero image to see the sqft-to-boxes conversion form I’m referring to, or navigate to any product on the site.

Look in the Snippets folder for the buy-buttons.liquid file.