Flowchart describing the main components of the GALSBI-SPS galaxy population model. Galaxy physical properties are sampled hierarchically from analytical distributions and conditional relations, as detailed in Sect. 3. The box colours reflect the hierarchy of dependencies. Blue boxes represent properties that are independently sampled, with the GSMF being the starting point of the sampling. Yellow boxes represent properties conditioned only on galaxy stellar mass and redshift (namely, the galaxy SFH, velocity dispersion, morphology, and AGN). Green boxes represent properties conditioned simultaneously on galaxy stellar mass, redshift, and SFH (galaxy metallicity history, dust attenuation, and gas ionisation). All sampled properties are then passed to PROSsc pect (red box) to generate galaxy SEDs and magnitudes. Together with the morphological properties they form the input catalogue for the forward-modelling. Each model component is described in its corresponding subsection of Sect. 3.