T-Mobile, AT&T, and Verizon each provide 5G deployed in Los Angeles, San Francisco, San Diego, Sacramento, and the major tech corridor between San Jose and San Francisco. The I-5 and US-101 highway corridors running the length of the state have 4G at all major cities and towns.
The I-80 over the Sierra Nevada has coverage at the main mountain towns. Highway 1 along the Big Sur coast has severe coverage gaps between the Monterey Peninsula and San Luis Obispo. Yosemite Valley has limited coverage at the main visitor center, with no signal in most of the park.
Death Valley has no coverage except near the Furnace Creek Visitor Center. Highway 395 on the Eastern Sierra from Bishop to Mammoth Lakes has 4G in towns. The Napa Valley has good 4G along Highway 29 through the main wine towns.
Lake Tahoe has 4G around the main resort areas on both the California and Nevada sides.