EURC and USDC are both fiat-collateralized stablecoins, by which the issuer (Circle) maintains a reserve to back the number of issued stablecoins. This reserve typically consists of: • Fiat currencies: EUR and USD, respectively • Very low-risk and highly liquid bonds (eg. government bonds)
You can see the transparency reports and reserve composition for USDC and EURC on Circle's transparency website at https://www.circle.com/transparency.