From my experience, most restaurants use Ole Hickory or Southern Pride style smokers, and do not trim their briskets prior to cooking. They smoke for ever how long, then wrap and hold hold them until ready to serve. They could easily sit for a day or longer in the hot hold before being served. This will help retain the moisture and make them tender. These tend to be your more reasonably priced BBQ places.
The authentic places that cook on wood only pits will buy choice or higher, not hold them as long, and typically charge a higher price to reflect this.