Some generation PG&E owns, some is in a shared vehicle split between PG&E and a group of private investors, some is owned by other utilities (Edison) and some is just privately owned. Remember that generation was built up over 70 years. Different plants at different times with different politics ends up with different ownership arrangements. They all bid into CAISO and independently choose to produce or not based upon prices. The generators sell futures contracts and PG&E (and Edison) buy them either in 1 day cycles or 15 minute cycles (almost always some of both).