  Whether or not $z \gtrsim 6$ quasars lie in the most massive dark-matter
halos of the Universe is still a subject of dispute. While most theoretical
studies support this scenario, current observations yield discordant results
when they probe the halo mass through the detection rate of quasar companion
galaxies. Feedback processes from supermassive black holes and dust obscuration
have been blamed for this discrepancy, but the impact of these effects is
complex and far from being clearly understood. This paper aims to improve the
interpretation of current far-infrared observations by taking into account the
cosmological volume probed by the Atacama Large Millimeter/submillimeter Array
Telescope and to explain the observational discrepancies. We statistically
investigate the detection rate of quasar companions in current observations and
verify if they match the expected distribution from various theoretical models,
once convolved with the ALMA field-of-view, through the use of Monte Carlo
simulations. We demonstrate that the telescope geometrical bias is fundamental
and can alone explain the scatter in the number of detected satellite galaxies
in different observations. We conclude that the resulting companion densities
depend on the chosen galaxy distributions. According to our fiducial models,
current data favour a density scenario where quasars lie in dark-matter halos
of viral mass $M_{\rm vir} \gtrsim 10^{12}~{\rm M_{\odot}}$, in agreement with
most theoretical studies. According to our analysis, each quasar has about 2
companion galaxies, with a [CII] luminosity $L_{\rm [CII]} \gtrsim 10^8~{\rm
L}_{\odot}$, within a distance of about 1~Mpc from the quasar.
