Correlation effects play an important role in determining the ground state of two-dimensional electron systems. In this talk, I will be presenting our experimental study of interaction-driven electronic phases in crystalline graphene Van der Waals heterostructures. We focus on two types of systems. The first is the monolayer graphene under a strong magnetic field, where the energy band is reconstructed into dispersionless Landau levels. Electron-electron interaction breaks the spin- and valley-degeneracy and generates a rich isospin phase diagram of the ground state. We probe the system with magnons, a collective excitation carrying spin, which allows us to detect insulating phases that cannot be distinguished with conventional charge transport measurements. I will introduce the experimental techniques and our observations of isospin phase transitions in fully and partially filled Landau levels. The second type we studied is the crystalline graphene multilayers. These systems feature Van Hove singularities in their energy bands, and correlation effects can be significant without applying an external magnetic field. Combining electrical transport and quantum capacitance measurement, we observed a variety of magnetic and superconducting phases. I will present these novel phenomena and the possible mechanisms.