California Vehicle Code §23152 makes it unlawful to operate a vehicle while under the influence of alcohol, drugs, or both. In general, a person is said to be impaired if their blood alcohol level is a .08 or over. The blood alcohol level is measured through the use of blood alcohol tests, which were taken through the use of a breathalyzer, blood tests or urine test. The blood alcohol level of .08 is just a standard, and will vary under different circumstances.
The statute states under (a) and (b) that a person may not drive under the influence of alcohol with a blood alcohol level over .08. But under other sections of the statute, the person need only be higher than a .04 blood alcohol level to be found too impaired to operate a vehicle. These statutes apply when the driver is operating an Uber, or a taxi, (Section (e)) as well as commercial vehicles (Section (d)).
Section (f) makes it unlawful to operate a vehicle while under the influence of drugs and Section (g) makes it unlawful for a person to operate a vehicle under the influence of both alcohol and drugs. Drugs are harder to assess at an arrest or traffic stop than the use of alcohol alone.