The corner trim gives a solid edge to fix the siding up to and it also gives some end protection to the end grain.
Gable end siding shiplap.
Gable end walls exterior is finished with lp smartside prepainted wood panel siding 4 8 sheets tyvek house wrap 7 16 osb attached to the 2 12 s on 16 centers.
Then add 20 on the gable to cover the waste of all those angle cuts 96 x 20 115 sq.
Picture of corner trim as installed.
The peak is the tricky part multiply half of the overall wall length 12 by the vertical distance from the wall height to the peak 8 to get the gable end s square footage 12 x 8 96 sq.
The inside covering of the gable end walls will eventually either be drywall or wood shiplap boards.
A three person team two installers working from staging who feed measurements to another worker at a cut station on the ground can side a large gable end in an hour or two.