UI Design is mainly a task for a UI Designer which studied e.g. Communication Design or Human Interaction.
User Interface Design always starts with a good concept that also leads to another topic called UX Design.
Most of the UI Designer start with creating scribbles of a rough interface with a pencil or probably on a Tablet. The creation process then takes place on a laptop with using programs like Sketch, Adobe XD, Figma, InVision, Adobe Photoshop or other tools.