We present a new suite of mock galaxy catalogs mimicking the low-redshift Universe, based on an updated halo occupation distribution (HOD) model and a scaling relation between optical properties and the neutral hydrogen (HI) content of galaxies. Our algorithm is constrained by observations of the luminosity function and luminosity- and colour-dependent clustering of SDSS galaxies, as well as the HI mass function and HI-dependent clustering of massive HI-selected galaxies in the ALFALFA survey. Mock central and satellite galaxies with realistic values of $r$-band luminosity, $g-r$ and $u-r$ colour, stellar mass and HI mass are populated in an $N$-body simulation, inheriting a number of properties of the density and tidal environment of their host halos. The host halo of each central galaxy is also `baryonified with realistic spatial distributions of stars as well as hot and cold gas, along with the corresponding rotation curve. Our default HOD assumes that galaxy properties are a function of group halo mass alone, and can optionally include effects such as galactic conformity and colour-dependent galaxy assembly bias. The mocks predict the relation between the stellar mass and HI mass of massive HI galaxies, as well as the 2-point cross-correlation function of spatially co-located optical and HI-selected samples. They enable novel null tests for galaxy assembly bias, provide predictions for the HI velocity width function, and clarify the origin and universality of the radial acceleration relation in the $Lambda$CDM framework.
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