Peruvian. It's a common type of Cereus that grows very large and very fast. Origin unknown, and unknown in the wild, but they're thick all through SoCal and Scottsdale.

Thanks for the name.

Yup, it's Cereus peruvianus, often called "apple cactus" or "Peruvian apple cactus". That's a nice one. I've planted several around my garden from purchased plants, and a tons of youngsters I started from seed, so I'm envious of the nice character and multiple stems on yours. Keep the pics coming! Happy gardening!