I feel you, OP. I’m from California too where people blanket at 60* and anything below 40* is reason to panic.
Now in live in Wisconsin and have to deal with reality.
The reality is horses do much much better in cold weather than we do. Read the link posted above. Minnesota has a lot of great research.
Our horses are out during the day and in (a non-heated, not tightly closed up barn) at night. I don’t even blanket until it’s going to be below freezing during the day. So that could mean horses are naked in their stalls into the 20s at night. (Obviously there are a few exceptions to the rule, like the thinner than I’d like TB, etc.) They do fine.
In a perfect world, I would like to throw a sheet on them below 50* and rainy, but most of our horses don’t have sheets, and they would be roasting under a blanket at that temp. They survive.
And my horse? Well she’s a little on the chubby side and handles winter well so she doesn’t even see a blanket until we are at least in the low 20’s during the day and nearing single digits at night.
As for the barns? Windows (in stalls) and doors (at the ends of the aisles) are still open until we are 0 or below or having a terrible storm. Some owners choose to shut their windows earlier, which is their choice, but I prefer the barn to be open. If the horses lie down, they can avoid any wind completely.
Turnout happens as long as footing is acceptable. (Twice in the last two years there has been too much ice to safety turn out.) Our pastures are on a hill, so if we are into the negative temps and windy, I might do shortened turnout in paddocks so they aren’t on the hill. The only time I said “no turnout, it’s too cold” was when the wind chill was below -40.
In summary: I can’t possibly imagine any weather in Tehachapi where I would lock anyone up. (Assuming everyone is in good health and handling the weather fine.) I might feed extra hay, since your horses aren’t used to these temps, but that’s about it. That being said, half how we care for our horses is actually for our horses, the other half is for us. So do what gives you piece of mind. I could understand closing up the barn at night when it’s below freezing, but I wouldn’t limit daytime turnout in this 40’s and 30’s weather CA is having. (Again, assuming everyone is doing well and has a sheet or blanket on for the rain.)