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That's one of the brilliant things about bittorrent. The standard clients support a feature whereby, if you are downloading a file, you also automatically share the parts you've already downloaded to anyone who requests them. Brilliant! It turns "normal users" into "node runners" by default.
This is, IMO, how nostr should have been designed. We have DECADES of P2P software to build on top of and so often nostr is just re-inventing the wheel. Among these softwares, some of which basically offer identical features to nostr, nostr is not more private, secure, or decentralized. And instead of building on and expanding existing networks, we instead decided to make a brand new one. If you're curious about these existing softwares, read about I2P, Freenet, Hyphanet, and the various darknet and mixnet softwares of the 2000s. etc.
In nostr, every user should be a node, but instead, for some reason, nostr devs decided to centralize things around relays. Notes are so small, even an entry-level android phone could store and route tens of thousands of them.