They are contributing to their ideas, providing insight to help them make decisions. A project manager will want to know how much time is needed and when to do

They are contributing to their ideas, providing insight to help them make decisions. A project manager will want to know how much time is needed and when to do user research, diary studies, usability sessions, surveys, wireframes, user stories, etc, etc. A UI designer will want to work with the UX professional to understand what the user goals are, the user journey is. The UI designer will then design a UI to satisfy that story. Marketers will communicate with the UX professional to give insight into the market needs, what competitors are doing, who the target customers are, and they work together to produce a coherent marketing message that matches the product. Developers will work with the UX designer to understand the user impact of design decisions, the goals they are achieving, how to format usable error messages that fit a wider consistent experience, making sure that the press of a button doesn't have a 5 minute delay. The UX professional is the user's stakeholder to make sure everyone is building something with them in mind. Without that everything is very siloed. So yes, when I say they work with them, I really do mean it. They don't instruct, they don't follow, they simply collaborate.