  Observational studies are finding stars believed to be relics of the earliest
stages of hierarchical mass assembly of the Milky Way (i.e., proto-Galaxy). In
this work, we contextualize these findings by studying the masses, ages,
spatial distributions, morphology, kinematics, and chemical compositions of
proto-galaxy populations from the 13 Milky Way (MW)-mass galaxies from the
FIRE-2 cosmological zoom-in simulations. Our findings indicate that proto-Milky
Way populations: i) can have a stellar mass range between
$1\times10^{8}<\mathrm{M}_{\star}<2\times10^{10}[\mathrm{M}_{\odot}]$, a virial
mass range between
$3\times10^{10}<\mathrm{M}_{\star}<6\times10^{11}[\mathrm{M}_{\odot}]$, and be
as young as $8 \lesssim \mathrm{Age} \lesssim 12.8$ [Gyr] ($1\lesssim z
\lesssim 6$); ii) are predominantly centrally concentrated, with $\sim50\%$ of
the stars contained within $5-10$ kpc; iii) on average show weak but systematic
net rotation in the plane of the host's disc at $z=0$ (i.e.,
$0.25\lesssim\langle\kappa/\kappa_{\mathrm{disc}}\rangle\lesssim0.8$); iv)
present [$\alpha$/Fe]-[Fe/H] compositions that overlap with the metal-poor tail
of the host's old disc; v) tend to assemble slightly earlier in Local
Group-like environments than in systems in isolation. Interestingly, we find
that ~60% of the proto-Milky Way galaxies are comprised by 1 dominant system
($1/5\lesssim$M$_{\star}$/M$_{\star,\mathrm{proto-Milky Way}}$$\lesssim4/5$)
and 4-5 lower mass systems (M$_{\star}$/M$_{\star,\mathrm{proto-Milky
Way}}$$\lesssim1/10$); the other ~40% are comprised by 2 dominant systems and
3-4 lower mass systems. These massive/dominant proto-Milky Way fragments can be
distinguished from the lower mass ones in chemical-kinematic samples, but
appear (qualitatively) indistinguishable from one another. Our results could
help observational studies disentangle if the Milky Way formed from one or two
dominant systems.
