What procedural steps should an adult in California follow in order to obtain full access to complete and uncensored records of court proceedings (in Contra Costa County, in the early 1970s) that led to his or her custodial birth parent losing all legal parental rights, with the child being placed into foster care?

I think the key thing here is that the adult requesting the records is the same person as that child of long ago, so any legal arguments about protecting the child's privacy should not apply.

The adult wants to know in particular what the testimony was, what records were entered as evidence, and so on, to understand why the birth parent was ruled to be an unfit guardian.