Seed discovery for bitcoin core client
Bitcoin Core chooses peers in the P2P network in a few ways:


it tries to connect to peers it already knows about, or
it connects to peers it learns about by querying volunteer-run DNS services, or
it connects to one of a number of hardcoded seeds 


Once connected to the P2P network, peers rumor possible other nodes they know about, and the node builds a picture of the network using these responses. On a fresh installation there's no known peers, so the discovery begins at the seed nodes.