  We investigate the formation of Milky-Way-mass galaxies using FIRE-2 LCDM
cosmological zoom-in simulations by studying the orbital evolution of stars
formed in the main progenitor of the galaxy, from birth to the present day. We
classify in situ stars as isotropic spheroid, thick-disc, and thin-disc
according to their orbital circularities and show that these components are
assembled in a time-ordered sequence from early to late times, respectively.
All simulated galaxies experience an early phase of bursty star formation that
transitions to a late-time steady phase. This transition coincides with the
time that the inner CGM virializes. During the early bursty phase, galaxies
have irregular morphologies and new stars are born on radial orbits; these
stars evolve into an isotropic spheroidal population today. The bulk of
thick-disc stars form at intermediate times, during a clumpy-disc ``spin-up''
phase, slightly later than the peak of spheroid formation. At late times, once
the CGM virializes and star formation ``cools down," stars are born on circular
orbits within a narrow plane. Those stars mostly inhabit thin discs today.
Broadly speaking, stars with disc-like or spheroid-like orbits today were born
that way. Mergers onto discs and secular processes do affect kinematics in our
simulations, but play only secondary roles in populating thick-disc and in situ
spheroid populations at z=0. The age distributions of spheroid, thick disc, and
thin disc populations scale self-similarly with the steady-phase transition
time, which suggests that morphological age dating can be linked to the CGM
virialization time in galaxies.
