  We study the structure of the accretion flow in the hard state of the
black-hole X-ray binary MAXI~J1820+070 with \nicer data. The power spectra show
broadband variability which can be fit with four Lorentzian components peaking
at different time scales. Extracting power spectra as a function of energy
enables the energy spectra of these different power spectral components to be
reconstructed. We found significant spectral differences among Lorentzians,
with the one corresponding to the shortest variability time scales displaying
the hardest spectrum. Both the variability spectra and the time-averaged
spectrum are well-modelled by a disc blackbody and thermal Comptonization, but
the presence of (at least) two Comptonization zones with different temperatures
and optical depths is required. The disc blackbody component is highly
variable, but only in the variability components peaking at the longest time
scales ($\gsim1$ s). The seed photons for the spectrally harder zone come
predominantly from the softer Comptonization zone. Our results require the
accretion flow in this source to be structured, and cannot be described by a
single Comptonization region upscattering disc blackbody photons, and
reflection from the disc.
