What makes California a Democratic state?

A big reason for Democratic stronghold in California is due to the top two candidates open primary act - proposition 14.

A new open primary law took effect in 2012. The “Top Two Candidates Open Primary Act” requires that all candidates for a voter-nominated office be listed on the same ballot. This means that a voter could cast his/her vote for any candidate, regardless of what party preference the voter indicated on his/her voter registration form. Only the two candidates receiving the most votes – regardless of party preference – would then go to the General Election ballot, regardless of vote totals.

https://ballotpedia.org/Top-two_primary

The top two vote-getters, regardless of their partisan affiliations, advance to the general election. Consequently, it is possible for two candidates belonging to the same political party to win in a top-two primary and face off in the general election. California followed suit in 2010.

So that means Republicans and Democrats could run in the primary, but if two Democrats get the most votes, then only Democrats will be in the general election. This is a positive feedback loop to a single party to maintain power. Even if a candidate is bad, an opponent would still run as the major popular party but with different policies.

Here is a history of party control in California.

https://ballotpedia.org/California_State_Assembly#History_of_partisan_control

This article from fivethirtyeight analyses California's shift from Republican to Democratic in depth. Some interesting passages:

Southern California was Republican-leaning, largely because the defense industry was a major economic engine [...] In the early 1990s, defense spending began to fall, and numerous military bases in California were closed. This helped spark an out-migration of mostly white, affluent and Republican-leaning residents, including many former defense-industry workers.

[...] The influx into California of Hispanics and Asians had an even larger effect. Between 1980 and 2000, California’s Hispanic and Asian communities each doubled as a share of the state’s population.

[...]Its main economic drivers were changing. Although agriculture remains a major industry, defense faded and Hollywood and Silicon Valley grew. California saw an influx of highly educated young professionals.

California is a coastal state that is urbanized compared to even other coastal states. It has more than its share of trade relationships, both domestic and international, and a diversity of people as a result. In this regard, it is much like the liberal Northeastern states in New England, and the "MidAtlantic" region. As such, its "natural" tendency would be Democratic.

The "puzzling" part is why California was Republican, at least at the level of Presidential politics for most of the period between 1952 and 1988. The answer appears to be the role of "native son" candidates on the Republican ticket. Californian Richard M. Nixon was the Vice-Presidential candidate (for Eisenhower) in 1952 and 1956, then the Presidential candidate in 1960, 1968, and 1972, with California voting Republican in those three years, but not in 1964. Then Californian Ronald Reagan led the Republican ticket in 1980 and 1984. He was also the "invisble man" for the Republicans in 1988, and IMHO, 1976. Beginning in 1992, no Californian came anywhere close to the Republican ticket, nor did the state vote Republican in Presidential elections since then.

Many people point to the overwhelming Republican support for (from the Republican Governor Pete Wilson on down), and passage of, Proposition 187 in 1994 as the catalyst that doomed the Republican party for a generation or more in statewide elections in California. The Proposition was an illegal immigration based ballot initiative that was similar in many ways to the recent controversial Senate Bill 1070 that passed in Arizona. It was intended to put in place a screening system to ensure that state services (education, healthcare, etc.) were not being used by those in the state illegally.

The measure was rejected by over 60% of Democrats in the state while Republicans and Independents supported it to a tune of 78% and 62% respectively. This dichotomy tied the Republican fates to the popularity of the bill after it was enacted. As protests mounted, and subsequent campaigns were run as a referendum on Proposition 187 (the sitting governor was replaced by Gray Davis, a Democrat, who ran a campaign explicitly against Proposition 187), the Republican brand in the state suffered as well. Coupled with significant demographic shifts that sees 38% of the state's population now identifying as Hispanic, the party simply could not overcome the rising tide of the Democrats. Indeed, since the passage of Proposition 187 Arnold Schwarzenegger is the only Republican to win a statewide election.