Flex has partial JS files for sections, I don’t have a copy of it locally but it looks likely to be in:
assets/scripts/sections/jsProduct.js
However I would be careful editing JS files if you are not familiar - the admin theme code editor does not store previous versions of JS files so it’s possible to break things quite easily with no way to revert. Have a read of this:
https://help.outofthesandbox.com/hc/en-us/articles/360033812234