  SnSe is a promising thermoelectric material with record-breaking figure of
merit, \textit{i.e., ZT}. As a semiconductor, optimal electrical dosage is the
key challenge to maximize \textit{ZT} in SnSe. However, to date a comprehensive
understanding of the electronic structure and most critically, the self-hole
doping mechanism in SnSe is still absent. Here, we report the highly
anisotropic electronic structure of SnSe investigated by both angle-resolved
photoemission spectroscopy and quantum transport, in which a unique
"\textit{pudding-mold}" shaped valence band with quasi-linear energy dispersion
is revealed. We prove that the electrical doping in SnSe is extrinsically
controlled by the formation of SnSe$_{2}$ micro-domains induced by local phase
segregation. Using different growth methods and conditions, we have achieved
wide tuning of hole doping in SnSe, ranging from intrinsic semiconducting
behaviour to typical metal with carrier density of $1.23\times 10^{18}$
cm$^{-3}$ at room temperature. The resulting multi-valley transport in $p$-SnSe
is characterized by non-saturating weak localization along the armchair axis,
due to strong intervalley scattering enhanced by in-plane ferroelectric dipole
field of the puckering lattice. Strikingly, quantum oscillations of
magnetoresistance reveal three-dimensional electronic structure with unusual
interlayer coupling strength in $p$-SnSe, which is correlated to the
interweaving of SnSe individual layers by unique point dislocation defects. Our
results suggest that defect engineering may provide versatile routes in
improving the thermoelectric performance of the SnSe family.
