with fixed prices or under an auction where the interested parties set the value of the NFT according to their own criteria

Under the auction, the owner of the nft sets his floor price. Now the floor price can be described as an opening bid just like a real auction in the real world. So users have to match that offer or bud higher to win the bid and buy the nft.

the exchange between two NFTs on digital drawings, both have different quality, dimensions, motifs, creativity, etc., so the exchange would be unequal and it cannot be divided since it is a finished product and its value depends on all its pieces, that is, it cannot be cut or divided to equalize the value between them.

That’s true but as you know it technology keeps on evolving.

Recently I heard one of the rich guys in the nft and crypto space say that he actually use an NFT as a collateral to get a loan in usdc. I think he said he Usdc loan was about 2 million dollars or more. But after some time he was able to acquire the loan to buy his dream how and he used the nft for that loan.

The exploitation of the NFT market is relatively new, so they have not yet undergone effective legislation regarding the regulation of their operation

Yep, I think the government have not placed their eyes on it yet and very soon they will be looking at ways to regulate the nft space and am pretty sure other countries will want tax on the nfts.