We study the formation of black holes from subhorizon and superhorizon perturbations in a matter dominated universe with 3+1D numerical relativity simulations. We find that there are two primary mechanisms of formation depending on the initial perturbations mass and geometry -- via $textit{direct collapse}$ of the initial overdensity and via $textit{post-collapse accretion}$ of the ambient dark matter. In particular, for the latter case, the initial perturbation does not have to satisfy the hoop conjecture for a black hole to form. In both cases, the duration of the formation the process is around a Hubble time, and the initial mass of the black hole is $M_mathrm{BH} sim 10^{-2} H^{-1} M_mathrm{Pl}^2$. Post formation, we find that the PBH undergoes rapid mass growth beyond the self-similar limit $M_mathrm{BH}propto H^{-1}$, at least initially. We argue that this implies that most of the final mass of the PBH is accreted from its ambient surroundings post formation.
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