There are a lot of messages that are sent around the Bitcoin network - e.g. updating new blocks, spreading new transactions. Do these not flood the network? Is it a pure broadcast to all the connected nodes?
Transaction and block broadcast use a gossip mechanism which floods the network. A node will send a transaction or block to its peers. Once those peers validate the transaction or block, those peers will send the transaction or block to their peers and so on and so forth. This results in all nodes receiving a transaction or block.