  The CONUS experiment is searching for coherent elastic neutrino nucleus
scattering of reactor anti-neutrinos with four low energy threshold
point-contact high-purity germanium spectrometers. An excellent background
suppression within the region of interest below 1keV (ionization energy) is
absolutely necessary to enable a signal detection. The collected data also make
it possible to set limits on various models regarding beyond the standard model
physics. These analyses benefit as well from the low background level of
~10d$^{-1}$kg$^{-1}$below 1keV and at higher energies. The low background level
is achieved by employing a compact shell-like shield, that was adapted to the
most relevant background sources at the shallow depth location of the
experiment: environmental gamma-radiation and muon-induced secondaries.
Overall, the compact CONUS shield including the active anti-coincidence
muon-veto reduces the background by more than four orders of magnitude. The
remaining background is described with validated Monte Carlo simulations which
include the detector response. It is the first time that a full background
decomposition in germanium operated at reactor-site has been achieved. Next to
remaining muon-induced background, $^{210}$Pb within the shield and cryostat
end caps, cosmogenic activation and air-borne radon are the most relevant
background sources. The reactor-correlated background is negligible within the
shield. The validated background model together with the parameterization of
the noise are used as input to the likelihood analyses of the various physics
cases.
