Not quite right.

The assets aren’t necessarily stored on OpenSea. You can see how OpenSea gets the metadata like asset url via the docs:

https://docs.opensea.io/docs/metadata-standards

If you scroll down you’ll see that the assets can be hosted on decentralised networks too like IPFS.

Also, not all NFT projects are built the same. I suggest you check out Pak’s Poets project - it’s a very exciting piece of performance art built around game theory.

Disclaimer: I don’t own any NFTs but that doesn’t mean I’m not excited by what they’re doing.