The real answer is that the cooks cooked with other cooks, who knew what they were doing, for a long enough time, that they also learned to do it right. For most of us, that is just not an option.
So, we come here, get ideas, cook and ruin meat and eventually get a grasp of the truth. For me, I like to start with a good cut of meat. Makes it easier to start with choice grade brisket packers. I prefer meat in the 12 to 14 pound trimmed range, so a 14 to 16 pound packer. I leave at least 1/8" of the fat cap. I trim the sides and edges so that they are no less than 1" thick. I cook the entire packer until the flat is done, probes quite easily. You can wrap of not, it will still come out good. I separate the flat, spray it with about 1/8 cup of beef broth or other flavorful liquid, wrap in foil and rest for 2 to 4 hours. The point goes back on, and cooks until it jiggles. it gets wrapped in foil and rests along with the flat.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"I'm Armenian...perhaps...but then again...maybe not..."