DarkSide-20k is a next-generation dual-phase Liquid Argon Time Projection Chamber, currently
under construction at the Gran Sasso National Laboratory (LNGS) in Italy. With a 20 t fiducial
mass of liquid Argon, DarkSide-20k will probe WIMP-nucleon interactions down to cross sections
equal to $10^{48}$ cm$^2$ for a WIMP mass of 0.1 TeV/c$^2$. DarkSide-20k is designed to be a nearly
“instrumental background-free” experiment, meaning that less than 0.1 background events are
expected in the WIMP search region during the 200 tonne-year planned exposure. To achieve
this, the TPC is surrounded by an inner (neutron) and outer (muon) veto, while low-radioactivity
underground argon (depleted in $^{39}$Ar), is used as the inner detector (TPC and inner veto) medium.
Both the TPC and the veto systems are instrumented with novel cryogenic silicon photomultipliers,
capable of resolving single photoelectrons and providing the required spatial and time resolution.
An overview of the DarkSide-20k experimental program is reported, with a focus on the photodetector
system construction and testing procedures for the inner veto system.