There are lots of technical and business tradeoffs that need to be balanced during the planning process of an agile project, and everyone needs to have their perspective included.

However, at the end of the day, someone takes notes, someone builds the "plan" that gets posted and communicated outwards (if relevant) -- this person is usually the SM, as the technical team members are busy sizing stories, doing design, etc. and the PO doesn't have the skills (nor the motivation) to do this. As for the Sprint Backlog, the data to build it and update it comes from the team, but the work to do the initial build or update is an administrative task that usually gets assigned to the SM - *unless you have a tool like VersionOne that makes the updates quick and easy for the team* - even though there are good tools out there, most people I've seen still end up using Excel spreadsheets for their backlogs.