Evolution of CG mean surface brightness in time relative to median threshold crossing. Shaded regions encompass the 25th-75th percentiles. Colours indicate samples identified at redshift z = 0.00 (blue), z = 0.06 (dark red), z = 0.14 (magenta), and z = 0.23 (green). The last point of each curve corresponds to the final snapshot, at t_cosmic ≈ 13.8 Gyr. The horizontal dashed red line shows the threshold ${\overline{\mu }}_r=26.33\mathrm{mag}{\mathrm{arcsec}}^{-2}$ and the vertical dotted black line indicates t_cosmic = t_threshold. When shifted horizontally, all medians follow a similar trend: galaxies begin to inhabit the same halo, leading to the group assembly, a period of extreme compactness, and subsequent orbital evolution.