  Four-dimensional compactifications of string theory provide a controlled set
of possible gauge representations accounting for BSM particles and dark sector
components. In this review, constraints from perturbative Type II string
compactifications in the geometric regime are discussed in detail and then
compared to results from heterotic string compactifications and
non-perturbative/non-geometric corners. As a prominent example, an open string
realization of the QCD axion is presented. The status of deriving the
associated low-energy effective action in four dimensions is discussed and open
avenues of major phenomenological importance are highlighted. As examples, a
mechanism of closed string moduli stabilization by D-brane backreaction as well
as one-loop threshold corrections to the gauge couplings and balancing a low
string scale $M_{\text{string}}$ with unisotropic compact dimensions are
discussed together with implications on potential future new physics
observations. For illustrative purposes, an explicit example of a globally
consistent D6-brane model with MSSM-like spectrum on $T^6/(\mathbb{Z}_2 \times
\mathbb{Z}_6 \times \Omega {\cal R})$ is presented.
