They are cities. San Mateo is a city. San Jose is a city. Hayward is a city. Redwood City is... a city. Union City is... a city. Oakland isn't just a city, it's the eighth most populated city in the whole of California!
(SJ native here)
https://en.wikipedia.org/wiki/Town#United_States
CA does not legally acknowledge any distinction between towns and cities. However, many states require a "town" to have fewer than 1,000, 2,000, 6,000, etc., residents (the bound varying by state). Union City has ~75,000 people, which would prevent it from being considered a small town in the majority of states that distinguish rigorously between towns and cities.