  We employ the hydrodynamical simulation IllustrisTNG-300-1 to explore the
halo occupation distribution (HOD) and environmental dependence of luminous
star-forming emission-line galaxies (ELGs) at $z \sim 1$. Such galaxies are key
targets for current and upcoming cosmological surveys. We select model galaxies
through cuts in colour-colour space allowing for a direct comparison with the
Extended Baryon Oscillation Spectroscopic Survey and the Dark Energy
Spectroscopic Instrument (DESI) surveys and then compare them with galaxies
selected based on specific star-formation rate (sSFR) and stellar mass. We
demonstrate that the ELG populations are twice more likely to reside in
lower-density regions (sheets) compared with the mass-selected populations and
twice less likely to occupy the densest regions of the cosmic web (knots). We
also show that the colour-selected and sSFR-selected ELGs exhibit very similar
occupation and clustering statistics, finding that the agreement is best for
lower redshifts. In contrast with the mass-selected sample, the occupation of
haloes by a central ELG peaks at $\sim$20\%. We furthermore explore the
dependence of the HOD and the auto-correlation on environment, noticing that at
fixed halo mass, galaxies in high-density regions cluster about 10 times more
strongly than low-density ones. This result suggests that we should model
carefully the galaxy-halo relation and implement assembly bias effects into our
models (estimated at $\sim$4\% of the clustering of the DESI colour-selected
sample at $z = 0.8$). Finally, we apply a simple mock recipe to recover the
clustering on large scales ($r \gtrsim 1 \ {\rm Mpc}/h$) to within 1\% by
augmenting the HOD model with an environment dependence, demonstrating the
power of adopting flexible population models.
