What does broadcast transactions/blocks to other neighbours/nodes mean exactly?

Learning about bitcoin I read things like “You create a transaction and broadcast it to your neighbours/nodes and then the nodes verify the signature and hash, mine the block and broadcast the block to other nodes.

I read bitcoin uses port 8333 for example. But how does each node know which other nodes to send its data to? I guess each node must have some list of other nodes to communicate with and API endpoints to send that list to other nodes that want to join the network.

I might be out in the wild here but nodes cannot possibly send arbitrary wifi signals out in the wind and at the same time listen to arbitrary wifi signals with some header “bitcoin” on them because isn’t that illegal right?

Also about how many other nodes do you send each transaction and block to?