  The transport of active particles may occur in complex environments, in which
it emerges from the interplay between the mobility of the active components and
the quenched disorder of the environment. Here we explore structural and
dynamical properties of Active Brownian Particles (ABPs) in random environments
composed of fixed obstacles in three dimensions. We consider different
arrangements of the obstacles. In particular, we consider two particular
situations corresponding to experimentally realizable settings. Firstly, we
model pinning particles in (non--overlapping) random positions and secondly in
a percolating gel structure, and provide an extensive characterization of the
structure and dynamics of ABPs in these complex environments. We find that the
confinement increases the heterogeneity of the dynamics, with new populations
of absorbed and localized particles appearing close to the obstacles. This
heterogeneity has a profound impact on the motility induced phase separation
(MIPS) exhibited by the particles at high activity, ranging from nucleation and
growth in random disorder to a complex phase separation in porous environments.
