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Blockchains are a network of nodes. Each has its own state(copy) of the current blockchain. Each node running Ethereum core protocol software that enforces the rules like PoW, Transaction validity etc. When we do a transaction (write) or read something from the blockchain we are basically interacting with one of these nodes from the network. But we need our own node to be able to talk to them. Metamask uses Infura (Node Service) whose nodes talk to these blockchain nodes for us. Hence, “Broadcasting” our transaction or sending it to the transaction pool (memPool).