My understanding is that Independence HS and Westmont HS are the default locations for the past few years for section finals.
In the event that there are two teams that are close teach other (Burlingame vs Carlmont) they will also use Sequoia or Terra Nova to the North or a venue on the other side of the hill if both teams are from Monterey, Santa Cruz, or San Benito counties. They also use SJCC sometimes if it is available and a larger crowd is expected. As someone mentioned above these venues are usually reasonably priced and available. I don't think they are waiting until the semi-finals are played and then scrambling to find facilities. They start with a basic plan in place.