  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. We use the power
spectra to reconstruct the energy spectra of the variability components peaking
at four different time scales. We find that the spectrum changes as a function
of time scales. The two variability components peaking at longer time scales
have similar shape, while the two peaking at the shorter time scales differ
significantly. In particular, the one corresponding to the shortest time scales
has 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 ($\geq1$ 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.
