Hi, I have an existing Web project that I developed in NextJS and is running in production. I am trying to create the mobile app out of it. So should I create a copy of the code that I developed for the web and work on it (installing dependencies and config for Capacitor) or I can do on my existing one?
I am not familiar with Next.js but you should be able to use the same code base. That is the whole point of Capacitor, one code base for everything 
Capacitor does have a Next.js template mentioned here, so it should work just fine - Capacitor Templates | Capacitor Documentation.