I don't know about California. In my state - which is not near california - it takes a court hearing/order to commit someone. An alleged incompetent has a right to a jury trial if they choose or can proceed with a bench trial (no jury) and they have the right to testify and present evidence and the right to have an attorney appointed (automatic here when the hospital files its notice of involuntary hold).
There is a statutory set up - a hospital can hold some one for up to X hours without a court order - to hold someone longer - the hospital/dr must petition the court for an order to do so.