This little offset at the base of the chimney is called a cricket. The purpose of it is to prevent leaking.
It diverts the water to the sides, away from the valley formed by chimney and roof.
The other thing he realized on Monday morning when he got up there to work was that he’d made it so high he would be unable to clean the chimney. So he put the plywood up to stabilize the verticals, then took 8” off the top and rebuilt the top.
Looking down the chimney to the 2 stove pipes coming out of the masonry heater. The one on the left is for the wood stove in the cellar, the other for the heater.
Tuesday he hopes to get the roofing back on and the sides weathertight.