  We have recently developed a method to kinematically decompose simulated
galaxies that helps to break the degeneracy among galactic stellar structures.
For example, the concept of stellar halos is generalized to weakly-rotating
structures that are composed of loosely bound stars, which can hence be
associated to both disk and elliptical type morphologies. By applying this
method to about 500 central galaxies with stellar mass $10^{10-11.5}\ M_\odot$
from the TNG50 simulation at $z=0$, we identify three broadly-defined types of
galaxies: ones dominated by disk, by bulge, or by stellar halo structures. We
then use the simulation to infer the underlying connection between the growth
of structures and physical processes over cosmic time. Tracing galaxies back in
time, we recognize three fundamental regimes: an early phase of evolution
($z\gtrsim2$), and internal and external (mainly mergers) processes that act at
later times. We find that disk- and bulge-dominated galaxies are not
significantly affected by mergers since $z\sim2$; the difference in their
present-day structures originates from two distinct evolutionary pathways,
extended vs. compact, that are likely determined by their parent dark matter
halos in the early phase; i.e., nature. On the other hand, normal elliptical
galaxies are typically halo-dominated, forming by external processes (e.g.
major mergers) in the later phase, i.e., nurture. This picture challenges the
general idea that elliptical galaxies are the same objects as classical bulges
in disk galaxies. In observations, both bulge- and halo-dominated galaxies are
likely to be classified as early-type galaxies with compact morphology and
quiescent star formation. However, here we find them to have very different
evolutionary histories.
