  We combine the latest spectrally stacked data of 21-cm emission from the
ALFALFA survey with an updated version of the Dark Sage semi-analytic model to
investigate the relative contributions of secular and environmental
astrophysical processes on shaping the HI fractions and quiescence of galaxies
in the local Universe. We calibrate the model to match the observed mean HI
fraction of all galaxies as a function of stellar mass. Without consideration
of stellar feedback, disc instabilities, and active galactic nuclei, we show
how the slope and normalisation of this relation would change significantly. We
find Dark Sage can reproduce the relative impact that halo mass is observed to
have on satellites' HI fractions and quiescent fraction. However, the model
satellites are systematically gas-poor. We discuss how this could be affected
by satellite--central cross-contamination from the group-finding algorithm
applied to the observed galaxies, but that it is not the full story. From our
results, we suggest the anti-correlation between satellites' HI fractions and
host halo mass, seen at fixed stellar mass and fixed specific star formation
rate, can be attributed almost entirely to ram-pressure stripping of cold gas.
Meanwhile, stripping of hot gas from around the satellites drives the
correlation of quiescent fraction with halo mass at fixed stellar mass. Further
detail in the modelling of galaxy discs' centres is required to solidify this
result, however. We contextualise our results with those from other
semi-analytic models and hydrodynamic simulations.
