The difference between UX and UI in few simple words, correct me if I'm wrong

The UX decides the words associated with every image/app etc, the pages and what information they should contain, what kind of images should be used along with the words, any information that should exist within the site and ofc tests the apps that are used within the site in order to see if they are usable

The UI decides the placement of everything, the exact images, the typography and colours

Basically UX adds information and UI organize it.

Correct me if I'm wrong but UX can work on paper too? why is there specific software for this type of job?
Also, is css mastery enough to become great at UI or do you need to learn photoshop?

Definitions will definitely vary depending on who you, or what company you look at. But, some common threads:

User Experiences focuses more on the theory and practicality of how to deliver the best experience for users. It's more technical and analytical. It may not have anything to do with design at all, at least not with any degree of fidelity. Rather, it's more about application flow, what information needs to be displayed versus what's superfluous. It focuses on identifying what needs to happen with an action takes place; what feedback is important or critical to ensure the user has the best experience.

User Interface is the design component. Not a subset of UX per se; the two are more ying and yang than parent/child. A UI designer focuses on sensory cues (typically visual, but not always). Ensuring what's on the page effectively conveys its message. The use of design as a method of user guidance and ensuring a smooth experience.

one is experience.....one is interface

UX is like the plan for a house
UI is like the elevation of the house