The Zapatistas are an example of "authoritarian" socialism, as you already explained. Democracy in socialism comes in many different forms, usually involving a combination of local voting and consensus gathering.

For China, public ownership is the principle aspect of the economy, and the CPC, a working class party, dominates the state. At a democratic level, local elections are direct, while higher levels are elected by lower rungs. At the top, constant opinion gathering and polling occurs, gathering public opinion, driving gradual change. This system is better elaborated on in Professor Roland Boer's Socialism in Power: On the History and Theory of Socialist Governance.

For the USSR, it was quite similar. First-hand accounts from Statesian journalist Anna Louise Strong in her book This Soviet World describe soviet elections and factory councils in action. Statesian Pat Sloan even wrote Soviet Democracy to describe in detail the system the soviets had built for curious Statesians to read about.

These are just 2 examples, but it extends to other socialist countries like Nicaragua, Venezuela, Cuba, the DPRK, Vietnam, Laos, etc, which all have their own unique conditions and systems in place. All use their systems of democracy to keep the working classes on top, while exerting authority to suppress capitalists, sabateurs, fascists, etc.