Yes, it’s a team responsibility, but it doesn’t mean that the PO can’t provide the first draft of the user story based on the stakeholders’ input. It’s hard to write a user story if you haven’t done any research with actual users and validated its value. In our team, PO writes the first draft of the user stories, and the team reviews them during the backlog refinement and sprint planning to make sure there is nothing missing. If there is, we are modifying the story or adding a new one. As for the technical debt effort which we include into the standalone stories, too, these are written by the tech expert (dev lead) and validated by the rest of the team.