I think they simply start different divisions on different days, at least according to the sailing instructions [1]
There are a variety of handicapping systems. This race uses TPYC ratings determined by US Sailing [2]
The TPYC rating uses the mainly downwind Transpac Wind Matrix, a Pacific Swell adjustment, a power trim adjustment, and a handicap course length of 2300 nautical miles to establish time allowances from the fastest rated yacht.
Other rating systems include IMS[3], attempting to make predictions of potential boat performance based on detailed hull measurements, and IRC[4], a committee based system trying to avoid attempts to game the handicaps.