Who are ETNs backed by?

Could you clarify for me, who exactly are the entities that provide the backing or underlying assets for Exchange Traded Notes (ETNs)? Are they similar to Exchange Traded Funds (ETFs) in that they track the performance of a particular index, commodity, or basket of assets? Or do ETNs operate differently, with their value determined by a different type of underlying asset or mechanism? Understanding the nature of the backing for ETNs is crucial in evaluating their potential risks and returns.