A transaction on the Ethereum blockchain is technically initiated by an external account owner (not a contract). For example, if User A sends 1 Ether (ETH) to User B, the action of debiting from one account and crediting to another changes the blockchain’s state.
The change specifically takes place on the Ethereum Virtual Machine (EVM). Ethereum transactions need to be broadcast to the entire network, and any node can broadcast a request for the execution of a transaction on the EVM.