Here again we have the PVC T fittings re-purposed as element to boom fittings.
The middle T fitting is reinforced with a bit of pipe glued in place and milled to a flat finish. Eventually a 5/8 inch hole will be drilled into this thicker piece to hold the “1/2 inch” copper pipe.
It is best to use a mill or good drill press with adjustable table to ensure the holes you drill are absolutely perpendicular to the axis of the T part of the fitting.