  FeSe is classed as a Hund's metal, with a multiplicity of $d$ bands near the
Fermi level. Correlations in Hund's metals mostly originate from the exchange
parameter \emph{J}, which can drive a strong orbital selectivity in the
correlations. The Fe-chalcogens are the most strongly correlated of the
Fe-based superconductors, with $d_{xy}$ the most correlated orbital. Yet little
is understood whether and how such correlations directly affect the
superconducting instability in Hund's systems.
  By applying a recently developed high-fidelity \emph{ab initio} theory, we
show explicitly the connections between correlations in $d_{xy}$ and the
superconducting critical temperature $T_{c}$. Starting from the \emph{ab
initio} results as a reference, we consider various kinds of excursions in
parameter space around the reference to determine what controls $T_{c}$. We
show small excursions in $J$ can cause colossal changes in $T_{c}$.
Additionally we consider changes in hopping by varying the Fe-Se bond length in
bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO$_{3}$
substrate (M-FeSe/STO). The twin conditions of proximity of the $d_{xy}$ state
to the Fermi energy, and the strength of $J$ emerge as the primary criteria for
incoherent spectral response and enhanced single- and two-particle scattering
that in turn controls $T_{c}$. Using constrained RPA, we show further that FeSe
in monolayer form (M-FeSe) provides a natural mechanism to enhance $J$. We
explain why M-FeSe/STO has a high $T_{c}$, whereas M-FeSe in isolation should
not.
  Our study opens a paradigm for a unified understanding what controls $T_{c}$
in bulk, layers, and interfaces of Hund's metals by hole pocket and electron
screening cloud engineering.
