Poseidon is headquartered in La Habra, CA (I've seen more than a few LA/OC cyclists riding Poseidons).

The Poseidon website says: "We are a consumer-direct business which means we don't go through any extra distribution channels. Our bikes are manufactured in a factory that we own and operate which allows us to ensure quality while giving you an affordable price. Our legacy of in-house manufacturing gives us the power and experience to source, design and build the highest-value bikes and components on the market." Don't know where the factory is located.

https://www.poseidonbike.com/pages/who-we-are