The state data of uncompressed NFTs is all stored in on-chain accounts. This is expensive at scale. Compressed NFTs save space by encoding the state data into an on-chain Merkle tree. The detailed account data is not stored on-chain, but in data stores managed by RPC providers. The Metaplex Digital Asset Standard (DAS) API represents a unified interface for interacting with digital assets on Solana, supporting both standard (Token Metadata) and compressed (Bubblegum) assets.
The API defines a set of methods that RPCs implement in order to provide asset data. In the majority of cases, the data is indexed using Metaplex Digital Asset RPC infrastructure.