Anyone can generate an NFT, including IP you don't own or existing collections.
Hundreds of wallets might contain a the same monkey picture (or same hash and IPFS link to nitpick).
What matters is that Opensea says you have the "real" one.
Their database is the real list of who owns what, the blockchain is a distraction.
You can see it in their anti-theft systems. NFTs get hidden and blocked from trading after a police report, even if it's still there on the chain.
I would disagree with that characterization. There are dozens of NFT marketplaces which all have access to the same underlying data. An NFT is denoted by its address on chain, which is trivial to find. Similarly anyone can create a website that looks like Google, but the "real" google is the DNS entry at "google.com".