What are the parts of an electric power steering and how do they work and interact together?

What are the parts of an electric power steering and how they work and interact together and if electric power steering goes bad can the steering wheel still be turned manually like it could when the power steering fluid of a regular power steering ran out, and also how much does it cost to replace an electric power steering system? I am the same guy who wrote about the air-conditioning system in my previous post, but I must ask one question at a time, thank you in advance for your valuable information.