In this talk, I will present a framework for constructing an infinite family of microstates for double-sided black holes in general relativity with a negative cosmological constant. These microstates, characterized by shells of matter located behind the horizon, offer new insights into the structure of the black hole Hilbert space, including its dimensionality. Using this approach, I will first demonstrate the factorization of the Hilbert space of double-sided black holes. I will then discuss how this framework can be applied to study out-of-equilibrium fluctuations of black holes. Finally, I will highlight several open problems that this framework now allows us to address.