Not much winterizing needed in SoCal, but I did change the furnace air filter and changed out the (rechargeable) AA batteries in the thermostat. We had some 40 F/5 C mornings in late November/early December, so the furnace (a Lennox installed Setp 2015) has actually fired up maybe half a dozen times. Works great. Keeping exterior doors shut and having double glazed windows really helps minimize heating use. I keep winter thermostat at 61 F, which I know is cold for some but feels fine to me. I like laying (vests, sweaters, etc.)
In late July/August, the AC is used perhaps 30 days a year. I keep thermostat set at 80 F. I am fine at 80 inside with fans going. The real goal of the AC is to keep my salt water aquarium from over heating above 80 F, as dissolved oxygen drops with rising water temperature.
I use Nordic Pure filters, MERV 12 value, obtained from Amazon. About $6-8 each, and I change them about every two months during heating or AC season. In my climate, AC season is late July to early September. By September, nights get rather cool, so if one opens the windows at night and closes them early in the morning, quite often AC use can be avoided. Heating season is late November to early March. So I change the filter three or four times a year.