Could you explain what you mean by "underlying"?
based on my experience in economics when currencies are printed to create a stable price that ensures the underlying and also the right financial circulation
is it like that in cryptocurrency?
Anyway, deflationary cryptocurrency (or crypto that is scarce or limited in supply) are designed to increase in price when demand for the crypto is more than supply.
Whereas National currencies, which are inflationary (or unlimited or abundant in supply), are designed to maintain fairly stable prices long-term whether demand increases or decrease.
If the cryptocurrency uses the underlying, will the cryptocurrency be more stable than other altcoins?
A deflationary cryptocurrency can certainly become as stable as national currencies if it has unlimited supply that is regulated to maintain its price long-term at certain price level... by increasing its supply when demand increases and decreases its supply when demand decreases.
in my opinion, if a cryptocurrency uses the underlying, the cryptocurrency has a fixed value based on the value of the underlying
for example if the underlying of project X uses the real underlying property and the supply keeps changing based on the underlying, will the altcoin be said to be a real asset and can it be legal?
Altcoin backed by physical(real?) assets will be dependent on the supply on the physical/real assets. It can only change in supply when the supply of the physical assets changes. It's simply backed by physical/real assets and can be considered legal.