NFT Questions

I'm fairly confused by the whole NFT stuff that is going on now.

If I purchase an NFT from OpenSea and the site goes under and ceases to exist… will the NFTs purchased from the site basically be worthless? They may still be referenced on a site like ipfs… but would you be able to transfer it to someone / sell it in the future?

What information is saved in an NFT. If I was to create an NFT for a photo that I want to sell, how does someone know what photo is being referenced? Is this information being saved on a public ledger or is it completely being saved within OpenSea's version of the Ethereum Blockchain?