  In the era of precision cosmology and ever-improving cosmological
simulations, a better understanding of different galaxy components such as
bulges and discs will give us new insight into galactic formation and
evolution. Based on the fact that the stellar populations of the constituent
components of galaxies differ by their dynamical properties, we develop two
simple models for galaxy decomposition using the IllustrisTNG cosmological
hydrodynamical simulation. The first model uses a single dynamical parameter
and can distinguish 4 components: thin disc, thick disc, counter-rotating disc
and bulge. The second model uses one more dynamical parameter, was defined in a
probabilistic manner, and distinguishes two components: bulge and disc. The
number fraction of disc-dominated galaxies at a given stellar mass obtained by
our models agrees well with observations for masses exceeding $
\log_{10}(M_*/M_\odot)=10$. S\'ersic indices and half-mass radii for the bulge
components agree well with those for real galaxies. The mode of the
distribution of S\'ersic indices for the disc components is at the expected
value of $n=1$. However, disc half-mass radii are smaller than those for real
galaxies, in accordance with previous findings that the IllustrisTNG simulation
produces undersized discs.
