Ownership is always defined in some context. Maybe you own a house, this ownership is declared and protected by your state or country. Or maybe you own a copyright to a song, this ownership is declared and protected by laws. Or maybe you own a t-shirt, this ownership exists in the realm of social norms and is protected by unspoken rules of being a reasonable person.
Ownership on web3 is similar, but instead of law or social norms, the rules are defined in code. I think it's very clever and has some properties that other ownership systems don't.
Law and social norms are constantly changing. There's a lot of nuance and gray areas. There's often an authority you can reach out to to change who owns what. For example, if you own a Gmail address, there is still an authority (Google) that can take it away or give it back to you if you got hacked.
Ownership on Ethereum is rigid and unforgiving.
Owning something on web3 means there's nobody, not even contract authors or Ethereum developers, that can break the rules to take it away from you.
The rules — the code that governs the tokens might have some unintuitive behaviors (people call them bugs), but there's nobody who has an authority to change things outside the rules of the system. If someone breaks the rules of the blockchain, it is no longer the same blockchain (and this indeed happened to Ethereum).
Ownership outside its original context falls apart. If you go to a radically different place (or time in history), things like owning a house, a song or even a t-shirt might no longer make sense.
Ultimately though, the concept of web3 ownership falls apart too when we move to the higher level of abstraction. All Ethereum code does is it says that this wallet address "owns" this token ID. Do you own a wallet if you know its seed phrase? Do you own a hardware wallet if you have it in your possession and know the PIN? What we make out of it is up to us, human and society.
What amazes me about NFTs is that this concept of ownership transitioned from being algorithmic rules to realms where it has a meaning in our social system. This is an extremely hard thing to pull off, and I see a lot of value in crypto because it managed to do it.