They use blockchain technology, the same used for Bitcoins and other cryptocurrencies, there are reports that one purchase with Bitcoin spends more electricity than your entire house for the entire month, if Bitcoin was a country, it would be spending more power than Argentina.

Plus, even if NFTs were not harming the environment, they are still stupid, you are using the blockchain technology to buy a PNG, JPG or GIF that you can easily download for free, and the only thing you get is a permanent record on the blockchain that you bought this therefore it's "yours" and it's "unique", it's like a very expensive autograph, this is only done because the art market is a scam, those terrible paintings that sell for millions are just money laundering and tax evasion, and they used NFTs to apply this to digital art.

Also! they can draw porn of your most obscure waifu!