(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.
Cities are urban. They have congestion, nightlife with noticeable quantities people on the streets in the evening, chance encounters as you walk around, and a general feeling of lots of stuff happening within a walkable distance.
https://en.wikipedia.org/wiki/Local_government_in_California...