In the Senate, they always seem to be addressing "Mister President" or "Madame President", and occasionally the presiding Senator says something - but it's never clear who that is (I listen to political news and speeches on the radio rather than TV, so I don't get to see the faces).
I don't think it's Orrin Hatch (Pro Tempore), at least not often - and it's rarely or never Mike Pence (VP/President of the Senate).
So I'm assuming Hatch or McConnell (Majority Leader) just picks someone to sit in the chair - do they rotate? Or is it whoever they feel like picking that day? Or do they draw straws?