Ratio between the mass of the spheroidal component and the total stellar mass (M_sph/M_⋆, which we refer as f_sph hereafter) as a function of the stellar mass (M_⋆). Grey dots show all the central galaxies with masses over 10¹⁰ M_⊙ in TNG100. The blue ellipses highlight our population of mostly-disc galaxies, selected to have a negligible spheroidal component; the green ellipses with a circle in the middle indicate our intermediate sample with f_sph value near the median of all the galaxies; and the red circles indicate the spheroid-dominated galaxies selected to have a negligible disc component at a given M_⋆. The dotted black line indicates the cut used to select the disc-dominated sample. The distribution of M_⋆ and f_sph for the selected samples are also shown at the top and left of the panel, respectively (blue solid lines for the mostly-disc dominated sample, green dot-dashed lines for the intermediate sample, and dotted red lines for the spheroid-dominated sample). The distribution of f_sph for all the central galaxies is also shown alongside the selected samples with a grey dashed line. The black dotted line in the f_sph distribution indicates the median of f_sph for all the galaxies. We use the same symbol, colour, and line-style convention as used in this figure to distinguish the different samples throughout all similar figures in the paper.