Of course there's no requirement to put all the data on the blockchain. Why would there be? I even consider torrent networks to be "web3" and they're not blockc

Of course there's no requirement to put all the data on the blockchain. Why would there be? I even consider torrent networks to be "web3" and they're not blockchains. It is not prohibitively expensive to store data on a blockchain, however. Arweave and Filecoin are the largest storage-centric blockchains. They each have multiple exabytes of data stored on them. Currently, you can store on Filecoin for free: https://nft.storage/ https://nft.storage/ as a promotional thing, but even for larger data stores (https://web3.storage/ https://web3.storage/) it's $10 / month for 120GiB and 8 cents per GiB after that. OpenSea itself is a marketplace with some centralized elements. That's fine. The core listing, selling and transfer logic is written as a series of smart contracts, however. They also compete with other marketplaces, some which are fully decentralized. They do not control or own the important data, which is the NFT itself. Some NFTs store all the metadata on chain, you can implement an NFT contract metadata as base64 encoded JSON (as you can see in a recent project https://etherscan.io/address/0x6f4388602c5dd6c593bf7c9cf3128aaa2a3e09ce#readContract https://etherscan.io/address/0x6f4388602c5dd6c593bf7c9cf3128..., plug in tokenId 2 into tokenURI to see). Some store them on IPFS with a hash based reference to the content. Some use centralized storage in a private database. All are valid use cases with varying applications. If you only want to own fully native-chain NFTs, feel free to filter based on that criteria.