how to deal with unused css and js ?

You should first use a theme, or builder such as Oxygen that is built specifically for max performance, by not loading unnecessary things.

Then use plugins like https://wordpress.org/plugins/freesoul-deactivate-plugins/ to remove plugins ability to “run” on the server, when accessing front-end pages, almost every wordpress website is doing this and it slows down a lot of things. This plugin also prevents entire plugins from loading on the front end, on pages which they are not required.

You will need to spend some time working with this out, no one can really tell you a simple way to do this, you have to simply understand more deeply how things work, and then build your website properly.