  Bulk FeSe becomes superconducting below 9\,K, but the critical temperature
(T$_{c}$) is enhanced almost universally by a factor of $\sim$4-5 when it is
intercalated with alkali elements. How intercalation modifies the structure is
known from in-situ X-ray and neutron scattering techniques, but why T$_{c}$
changes so dramatically is not known. Here we show that there is one-to-one
correspondence between the enhancement in magnetic instabilities at certain
$\bf q$ vectors and superconducting pairing vertex, even while the nuclear spin
relaxation rate ${1}/{(T_{1}T)}$ may not reflect this enhancement.
Intercalation modifies electronic screening both in the plane and also between
layers. We disentangle quantitatively how superconducting pairing vertex gains
from each such changes in electronic screening. Intercalated FeSe provides an
archetypal example of superconductivity where information derived from the
single-particle electronic structure appears to be insufficient to account for
the origins of superconductivity, even when they are computed including
correlation effects. We show that the five-fold enhancement in T$_{c}$ on
intercalation is not sensitive to the exact position of the d$_{xy}$ at
$\Gamma$ point, as long as it stays close to E$_{F}$. Finally we show that
intercalation also significantly softens the collective charge excitations,
suggesting the electron-phonon interaction could play some role in intercalated
FeSe.
