Here on four saw horses sits the frame for the tree house platform.
The joinery is a series of rabbets cut on the tablesaw. (Which is really the first time I've had to break in the new saw. Worked well. Good power. Quiet.) There'll be two other beams across the narrow section. Then those four beams will sit on the main supports.
The boards are old hardwood planks the previous owner was using for shelving. Machined, they look rather nice. Looks like blackbutt.