One may also note that Blockchain is not a P2P technology per se, but rather just a data-structure designed to reach global consensus provided with transactions acquired from a P2P network. Validation rules which govern the updates to this data-structure are consensus rules.

The actual Bitcoin P2P layer is similar to other P2P protocols(Gnutella, IPFS etc.), and is a messaging protocol designed to acquire and propagate public data with peers. Theoretically, a validating Bitcoin full-node may acquire its transaction/block updates from another channel/P2P protocol and still be in consensus.

There are many p2p application. One prominent one is Bittorrent (https://en.m.wikipedia.org/wiki/BitTorrent) which allows users to store, distribute and download files from peers. Bittorrent at some point made up more than half of the internet‘s traffic. Skype also used to be a P2P application but has since switched to a server based model.