Communism is a bit more nebulous, so I’ll explain it with socialism if that’s okay. I’m also going to do so from the ground up, because it’s pretty clear if you go step by step, so apologies in advance if it comes off as condescending for that reason.

Socialism is the common ownership of the means of production. The means of production refers to whatever creates goods and provides services.

If you think about what falls under the umbrella of goods and services, it’s pretty much everything. Food, water, shelter, health care, all of it should be held in common. You can see totalitarianism emerging already, but I’ll expand a bit further.

What does held in common mean? Well in theory it means that a collective takes ownership and control over something. If a state turns socialist in the purest sense, then that state and the people in it are the collective. The “common” would mean the ruling socialist party, of which everybody is a member.

So how do people get things done? Well, if someone owns a restaurant, they probably hire managers to keep order and give tasks to the workers. If a collective owns a restaurant, someone still has to decide who manages (you find out immediately that you need them if you try without). Since the state is the collective, then the state decides who the managers are. The workers may not like the managers, but instead of having a single owner to deal with, you now have the entire state to deal with.

You might think a union would be the answer to this problem, but unions are both a collective and a service, so if it’s truly socialism it would be of the state as well. The state would have little incentive to act against itself. In socialist countries, it’s common for unions to become agents of the state very quickly, this enacting state level control over how people protest their work conditions

Rinse and repeat with every good and service you can think of, and you have total state control over basically everything.