  Transition metal dichalcogenides offer unprecedented versatility to engineer
2D materials with tailored properties to explore novel structural and
electronic phase transitions. In this work, we present the atomic-scale
evolution of the electronic ground state of a monolayer of
Nb$_{1-\delta}$Mo$_{\delta}$Se$_2$ across the entire alloy composition range (0
< ${\delta}$ < 1) using low-temperature (300 mK) scanning tunneling microscopy
and spectroscopy (STM/STS). In particular, we investigate the atomic and
electronic structure of this 2D alloy throughout the metal to semiconductor
transition (monolayer NbSe$_2$ to MoSe$_2$). Our measurements let us extract
the effective doping of Mo atoms, the bandgap evolution and the band shifts,
which are monotonic with ${\delta}$. Furthermore, we demonstrate that
collective electronic phases (charge density wave and superconductivity) are
remarkably robust against disorder. We further show that the superconducting TC
changes non-monotonically with doping. This contrasting behavior in the normal
and superconducting state is explained using first-principles calculations. We
show that Mo doping decreases the density of states at the Fermi level and the
magnitude of pair-breaking spin fluctuations as a function of Mo content. Our
results paint a detailed picture of the electronic structure evolution in 2D
TMD alloys, which is of utmost relevance for future 2D materials design.
