What makes Bitcoin's network robust despite having "minimal structure"?

Messages are best-effort broadcasts, nodes need no identity, and any node can leave and rejoin — catching up simply by accepting the longest proof-of-work chain as what happened while it was gone.

The network deliberately assumes very little:

• Best-effort delivery — a dropped block is re-requested when the next one arrives; transaction broadcasts needn't reach every node.
• No identities — messages aren't routed anywhere in particular, so nodes don't need to be known or trusted.
• Free join/leave — a returning node just adopts the longest chain as the truth it missed.

Nodes coordinate almost not at all; they "vote" only by extending valid blocks and refusing to work on invalid ones. That unstructured simplicity is precisely what makes the system resilient.

Go deeper:

• Bitcoin network — Wikipedia — the best-effort, identity-free gossip network and how returning nodes catch up on the longest chain.