In most of the BigCommerce templates, the add to cart button is present by default. This is most of the time is against the quantity selector box. If you want to change the add to cart button from its normal position to a different position than one of the easy and quick fixes are move the add to cart button below the quantity box and for that, you can follow the following steps to get that done
You have to go to the template file from your BigCommerce Admin panel and then open the snippet folder
Once you are in, look for the file name productaddtocart.html in the snippet folder and click on it to open
Then add couple of breaks which is “<br />”
Now your product page will have the add to cart button option below the quantity selector.