The IceCube observatory at the South Pole is currently the world's largest neutrino detector, with roughly one cubic kilometre of instrumented ice volume. The observatory has detected high-energy astrophysical neutrinos and has provided evidence for the first neutrino point sources. To extend its energy range and sensitivity, plans are in place for the IceCube Upgrade, which will include the deployment of seven additional strings in the 2025/26 Antarctic summer season. This will pave the way for the next-generation detector, IceCube-Gen2, which will increase the volume to eight cubic kilometres and the detection rate of cosmic neutrinos by a factor of ten. This thesis presents studies on the characterisation of the novel multi-PMT Digital Optical Module (mDOMs) that will be one of the primary devices of IceCube Upgrade. These studies are based on simulations, measurements on bare mDOM photomultiplier tubes, and tests with the first built mDOMs.