When you say UX agency, you usually mean a studio or project company that provides services to other companies in the fields of user experience (UX). User experience is a field in the worlds of design, which emphasizes the human experience and the psychological aspects of using a certain digital product, along with its business needs.
UX agencies do a wide variety of projects, for example mobile and computer applications, complex systems, information kiosks and a variety of other products. UX agencies can be building a product from scratch, building a certain feature in an existing product and redesigning an existing one. A UX project will include part or all of the UX stages: research (organizational research, field research, user research, competitor research, usability studies), product design (information architecture, usage scenarios, logical design) and user interface design (color, shapes, fonts, etc.).
In these companies, the user experience (UX) architectures and designers will usually carry out the research, the characterization and the logical design of the product, and the user interface (UI) designers will carry out its visual design. However, you can also see companies that combine the roles when the product designer (UX/UI/Product Designer) will perform both the characterization work and the design.