Nostr notes are just self-contained packets of data with a verifiable sender. You can transfer Nostr messages over any interface. Wifi, Ethernet, Bluetooth, NFC, satellite, smoke signals, etc...
Let's say you have three different devices, A B and C, that you've configured to all communicate with one another using Nostr notes to share a single clipboard. Each device knows the pubkey of the other two, and can send and receive clipboard updates (which are NIP-04 DMs or something similar that's dual encrypted). If the user copies some text on device A, the device reaches out on a best-effort basis to device B and device C and sends a clipboard update message. It doesn't matter how those messages reach device B or device C, they just need to get there. If device A is with you at the office but device B is at home, using Wifi to post the message to a relay makes the most sense. If device A and device B are both with you on a road trip where there's no cell data, they can share Nostr notes through Bluetooth. Since these messages are ephemeral and only need to go from one device to another, the concept of a relay isn't even really needed for peer-to-peer sharing.