Peers exchange announcements
Nodes find peers through configured addresses, DNS seeds and addresses learned from the network. Inventory and related messages advertise objects before data is requested. Receiving the same block from many peers does not waive local validation.
Compact blocks reuse known data
BIP 152 sends a header, short transaction identifiers and selected prefilled transactions. The receiver reconstructs most of the block from its mempool and requests missing items; differing mempools can require another round or the full block.
Encrypted transport has a boundary
BIP 324 v2 protects P2P message contents and makes passive identification and active tampering harder. Ordinary connections do not prove a civil identity and the network layer still exposes endpoints such as IP addresses.
Connection diversity matters
An eclipse attacker tries to control all information reaching a node. Diverse inbound and outbound peers, networks and address groups make isolation harder. Encryption alone cannot help if the attacker is the peer at the end of the connection.