Simply put, what underpins NFTs is a Distributed Ledger Technology (DLT) that is almost impossible to tamper with. The more decentralized such a network is, the more difficult or impossible it is to hack into the data stored there.
For example, Bitcoin is backed by the strongest computational network in the world. However, there are many other distributed accounting technologies.
The interesting thing is that, in addition to the fact that the most well-known networks are very reliable, anyone with Internet access and a little knowledge can check the entire transaction history of Bitcoin, Ethereum, Polygon, BNB Smart Chain, as well as a given NFT.
Outside the world of digital, a person buying a piece of art can receive a certificate of authenticity. But this paper can be lost or destroyed.
In contrast, the authenticity of an NFT cannot be lost or tampered with. Moreover, the information confirming its authenticity is not hidden in a safe, but is visible and anyone can check not only its veracity, but also its time of creation, who created it, whether it was sold, at what price...
The backing of the NFTs does not depend on what a specialist or a well-intentioned person says, but on a distributed accounting technology that could hardly be hacked or altered by a group of humans.
As of today, the best known distributed accounting technology is the blockchain - which is the technology on which, for example, Bitcoin runs.
The blockchain is, therefore, a decentralized and distributed technology that allows digital information to be recorded in a network made up of several computers located in different parts of the planet, making it very difficult to alter this record, which is visible to everyone.
NFTs are created on different DLTs. Therefore, when someone creates, buys, sells, gives away or changes the portfolio of an NFT, a new record is generated in the DLT technology on which that NFT runs.
Therefore, if someone takes a screenshot of the image of an NFT, they are not creating a copy of the NFT nor do they own the NFT. Of course, a screenshot does not affect the blockchain or the owner of the NFT.