It is a matter of choice how one decides how they want their pool winterized.
The worry free method is to start with a perfectly chemically balanced pool and then blow all the water out of the equipment, skimmer and return lines and cap them with duck plugs or winterizing plugs. A bit more involved is if solar is a part of heating system. Then all the equipment is unhooked and drained of all water. The electrical and gas supply are also turned off. A winter blanket is then placed over their pool held in place with water bags, landscape ties, or in case of above ground pool the cover is held in place with a cable that is tightened with a winch. The only thing one has to do till spring opening is to drain rain water off of cover to reduce cover stretching and damage.
The other method and becoming more popular is to keep the pool running with no cover, or in some cases where there is an automatic or safety cover which can be left on throughout winter. Bypass the heater and drain, but keep water chemically balanced keeping close eye on PH and alkalinity. Adjust as needed – at least once a month. Maintain water level by draining to waste as the rain fills pool up. Rain on the west coast is high in acid and that is main reason why PH is so important to keep in check. If PH drops too low it will start stripping copper off your heat exchanger if not bypassed, and compromise life of heater and the chemistry of pool. It is also beneficial to vacuum once in a while so that leaves and debris do not stain liner or concrete.