In this talk, we describe a novel, universal phenomenon that occurs in strongly interacting many-body quantum dynamics beyond the conventional thermalization. The observed universality leads to the development of a novel benchmarking method applicable for a wide variety of near-term quantum devices. More specifically, we point out that a single many-body wavefunction can encode an ensemble of a large number of pure states defined on a subsystem. For a wide class of many-body wavefunctions, we show that the ensembles encoded in them display universal statistical properties by using a notion in quantum information theory, called quantum state k-designs. The special case (k=1) reduces to the conventional quantum thermalization. The universality is corroborated by two theorems, solvable models, extensive numerical simulations of Hamiltonian dynamics, and recent experimental observations based on a Rydberg quantum simulator. Our results offer a new approach for studying quantum chaos and provide a practical method for sampling pseudorandom quantum states. As an example of practical utility, I will explain how our results allow us to develop a novel sample-efficient benchmarking protocol, which has been already demonstrated in an experiment.