  Massive black hole (BH) mergers will be key targets of future gravitational
wave and electromagnetic observational facilities. In order to constrain BH
evolution with the information extracted from BH mergers, one must take into
account the complex relationship between the population of merging BHs and the
global BH population. We analysed the high-resolution cosmological
radiation-hydrodynamics simulation OBELISK, run to redshift $z=3.5$, to study
the properties of the merging BH population, and its differences with the
underlying global BH population in terms of BH and galaxy properties. In
post-processing, we calculated dynamical delays between the merger in the
simulation at the resolution limit and the actual coalescence well below the
resolution scale. We find that merging BHs are hosted in relatively massive
galaxies with stellar mass $M_\ast\gtrsim10^9\,M_\odot$. Given that galaxy mass
is correlated with other BH and galaxy properties, BH mergers tend to also have
a higher total BH mass and higher BH accretion rates than the global population
of main BHs. These differences generally disappear if the merger population is
compared with a BH population sampled with the same galaxy mass distribution as
merger hosts. Galaxy mergers can temporarily boost the BH accretion rate and
the host's star formation rate, which can remain active at the BH merger if
sub-resolution delays are not taken into account. When dynamical delays are
taken into account, the burst has generally faded by the time the BHs merge. BH
spins are followed self-consistently in the simulation under the effect of
accretion and BH mergers. We find that merging BHs have higher spins than the
global population, but similar or somewhat lower spins compared to a
mass-matched sample. For our sample, mergers tend to decrease the spin of the
final BH remnant.
