How to start a user journey map for a new product and how does it translate to a product backlog?

I'm working on a new product that's a talent marketplace for the events and entertainment space. The client has a rough idea (like 20%) of what the user journey would be like.

I'm curious how should i go about setting up the foundations for a user journey map. Right now i've identified 3 personas that would use the product for different reasons and i'm guessing there will be 3 user journeys.

Should i brainstorm with the client to build this journey? If so what would be a suitable brainstorming method? Also how could i validate this journey with real users?

Lastly, as the user journey gets more detailed, how this would connect to the product roadmap or product backlog?