User experience (UX) design focuses on designing which screens an app or a website should have. The process involves a lot of technical and analytical thinking and may involve conducting user studies and collecting data on how people think and operate within the field to determine common user flows.
Have you ever seen a 5 year old pick up an iPad game and just.. start playing it? That effect is the work of a UX designer, one who takes the controls, elements, and buttons, and makes it intuitive to use. Ever since mobile apps became mainstream, UX Designer became a genuine title and field of study, and the discipline extends into many other industries.
Pretty much everything from car dashboards to airport signage involve some kind of critical UX thinking into the best way of communicating the intended message. We wouldn’t want anyone to unconsciously drive faster at a primary school because of unclear signage.
There is some overlap with a similar discipline called User Interface (UI) Design but simply, UX and UI is the difference between a car being fantastic to drive and a car looking fantastic. They’re two sides of the same coin. Sometimes choosing which colour a button should be is about choosing which colour users would easily understand. For example 'continue' type buttons are usually green where as error and delete type buttons are red.
That isn't really a UI design choice but a user experience choice. A UX designer has to be very good at their job to make a website with the buttons switched and not confuse anyone.