Typically, the facilitator (with input from the leader and the secretary) prepares an agenda draft. At the beginning of the meeting, the facilitator presents the agenda and asks for the circle for their consent. (This is a normal consent decision, i.e. there could be objections, amendments etc.)
The agenda can be changed during the meeting (for instance if we run out of time), but this, again, has to be decided by consent.
What is being talked about and what is not is a decision that involves power. It makes sense to have someone (facilitator) pay attention to what needs to be talked about, but ultimately, the group makes the decision on what the agenda is. That way, equivalence and transparency can be supported in the decisions on what is relevant to spend time on and how much time we devote to each item.