I do not know or I’m not sure which .CSS and .JS files to unload on my pages or which JS files should have the “defer” and “async” attributes. What should I do?

If that’s the case, then it’s advisable to consult with a developer (ideally the person who helped you with your website) to give you assistance in unloading the unused assets.

Adding “defer” and “async” attributes to the JavaScript tags is an optional matter, but in some situations, it could considerably improve the speed of your pages. You can read more about it here: https://gtmetrix.com/defer-parsing-of-javascript.html