The simplest way to think about User Interface (UI) Design it is to think of it as the design of all the elements that users interact with on a screen. Think of all the Buttons, Labels, Text Fields, Controls, Switches, Sliders etc and then the layout of them.
Taking a button as an example, important things to consider in UI Design are:
Consistency is important (you don't want users to constantly guess what is a button on each screen) so usually this involves creating a Design System to ensure this.
If you have a project in mind that you would like to collaborate on, I'd love to hear about it!