When purchasing an NFT with fiat the NFT is held in a paper wallet.
It is extremely important that you do not lose this PDF!
This is your sole proof of ownership until you transfer it to a managed wallet.
Perhaps one of the best ways to treat it is to print it and place it somewhere safe.
It is highly encouraged that you send the NFT to an existing Cardano wallet (such as Eternl, Nami, etc).
To learn more about paper wallets for storing NFTs and other digital assets (such as cryptocurrency) read *here*