  We develop a hydrodynamic description of transport properties in
graphene-based systems which we derive from the quantum kinetic equation. In
the interaction-dominated regime, the collinear scattering singularity in the
collision integral leads to fast unidirectional thermalization and allows us to
describe the system in terms of three macroscopic currents carrying electric
charge, energy, and quasiparticle imbalance. Within this "three-mode"
approximation we evaluate transport coeffcients in monolayer graphene as well
as in double-layer graphene-based structures. The resulting classical
magnetoresistance is strongly sensitive to the interplay between the sample
geometry and leading relaxation processes. In small, mesoscopic samples the
macroscopic currents are inhomogeneous which leads to linear magnetoresistance
in classically strong fields. Applying our theory to double-layer
graphene-based systems, we provide microscopic foundation for phenomenological
description of giant magnetodrag at charge neutrality and find magnetodrag and
Hall drag in doped graphene.
