  The IllustrisTNG simulations reproduce the observed scaling relation between
stellar specific angular momentum (sAM) $j_{\rm s}$ and mass $M_{\rm s}$ of
central galaxies. We show that the local $j_{\rm s}$-$M_{\rm s}$ relation ${\rm
log}\ j_{\rm s} = 0.55 \ {\rm log}\ M_{\rm s} + 2.77$ develops at $z\lesssim 1$
in disk-dominated galaxies. We provide a simple model that describes well such
a connection between halos and galaxies. The index 0.55 of the $j_{\rm
s}$-$M_{\rm s}$ relation comes from the product of the indices of the $j_{\rm
tot}\propto M_{\rm tot}^{0.81}$, $M_{\rm tot}\propto M_{\rm s}^{0.67}$, and
$j_{\rm s}\propto j_{\rm tot}$ relations, where $j_{\rm tot}$ and $M_{\rm tot}$
are overall sAM and mass of a halo. A non-negligible deviation from the tidal
torque theory, which predicts $j_{\rm tot}\propto M_{\rm tot}^{2/3}$, should be
included. This model further suggests that the stellar-to-halo mass ratio of
disk galaxies increases monotonically following a nearly power-law function
that is consistent with the latest dynamical measurements. Biased collapse, in
which galaxies form from the inner and lower sAM portion of their parent halos,
has a minor effect at low redshifts. The retention factor of angular momentum
reaches $\sim 1$ in disk galaxies with strong rotations, and it correlates
inversely with the mass fraction of the spheroidal component, which partially
explains the morphological dependence of the $j_{\rm s}$-$M_{\rm s}$ relation.
