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The unprecedented tunability of van der Waals materials enable charge density to be modulated over a large range. In the first half of the talk, I will focus on superconductivity and insulating states in twisted bilayer graphene. In a magic angle device, we demonstrate that the superfluid stiffness is dominated by the quantum geometric contribution. In devices with larger twist angle, we observed dominant insulating states at 1/3-fractional fillings, which are accounted for by a strong-coupling theory of symmetry-breaking of geometrically frustrated fractional states. In the second half of the talk, I will discuss transport in a 2D semiconductor in the regime of high charge density, revealing giant tunable intersubband transition and quantum Hall ferromagnetism.