  In this thesis we study String Theory compactifications to four dimensions
focusing on the moduli stabilization process and the associated vacua structure
in various frameworks, from Type IIA to F-theory, interpreting the results in
the context of the Swampland Program. More specifically, we generalize the
bilinear formalism of the scalar potential to include the contributions of
geometric fluxes, which we use to perform a systematic search of vacua. We also
consider the 10d uplift of AdS4 vacua arising from the 4d massive Type IIA
effective theory with only RR and NSNS fluxes. Using the language of
SU(3)xSU(3) structures and performing an expansion around the smearing
approximation in powers of the string coupling, we study the stability of the
SUSY solution and its non-SUSY partner. We contrast the results with the Weak
Gravity Conjecture and the AdS instability conjecture in toroidal orbifold
examples and find that some non-SUSY cases are in tension with the predictions
of those conjectures. From the F-theory perspective, we study moduli
stabilization in the complex structure sector of elliptically fibered
Calabi-Yau 4-folds in the Large Complex Structure limit. Using homological
mirror symmetry, we replicate the analysis for the Type IIA case and give a
bilinear expression for the scalar potential, allowing for a detailed study of
the vacua structure. We find two distinct families of flux configurations
compatible with the tadpole constraints that enable full moduli stabilization.
We thoroughly examine the most generic one in the Type IIB limit, where the
superpotential is also quadratic and polynomial corrections can be considered
at all orders. Finally, we show that at this level of approximation
supersymmetric SUSY vacua always contain flat directions. We conclude with a
summary of the results and some comments about open questions and future lines
of research.
