Primary names (reverse records) let dApps display a human-readable name like vitalik.eth instead of a raw Ethereum address.
Primary names are set per-chain with a default fallback.
addr.reverse is the original ENSIP-3 namespace for Ethereum mainnet. Its resolver does not fall back to default.reverse.80002105.reverse (Base).default.reverse is a default fallback introduced in ENSIP-19.