What network is Optimism on?

Could you please explain to me the network that Optimism operates on? I'm trying to understand the technical aspects of this cryptocurrency project and its underlying infrastructure. Is Optimism built on Ethereum, or does it have its own independent blockchain? Additionally, how does the network architecture of Optimism contribute to its scalability and functionality? Could you also elaborate on the security measures implemented by the network to ensure the safety of transactions and assets? I'm keen to learn more about this fascinating topic. Thank you for your assistance.