> I mean, opensea.io does allow you to mint (the term for magically turning a media into an official NFT backed by the blockchain) up to 100MB of content.
So where is it? Not in the blockchain, obviously, it's already fat enough with tiny transaction data.
Well, it's just stored in the centralized platforms like opensea, rarible, etc.
You’re right that this is the way many have been created so far, but that will not be the case for long. Already people have pointed NFTs to IPFS/Filecoin which will give them much more staying power. And this can be done without using a centralized platform like opensea, check out nft.storage and metaplex (1) as starting points.
1. https://github.com/metaplex-foundation/metaplex