  We present results from quasi-simultaneous multiwavelength observations of
the Galactic black hole X-ray transient MAXI J1820$+$070 during the decay of
the 2018 outburst and its entire subsequent mini-outburst in March 2019. We fit
the X-ray spectra with phenomenological and Comptonizaton models and discuss
the X-ray spectral evolution comparing with the multiwavelength behaviour of
the system. The system showed a rebrightening in UV/Optical/NIR bands 7-days
after the soft-to-hard transition during the main outburst decay while it was
fading in X-rays and radio. In contrast, the mini-outburst occurred 165-days
after the hard state transition of the initial outburst decay and was detected
in all wavelengths. For both events, the measured timescales are consistent
with those observed in other black hole systems. Contemporaneous hard
X-ray/soft $\gamma$-ray observations indicate a non-thermal electron energy
distribution at the beginning of the UV/Optical/NIR rebrightening, whereas a
thermal distribution can fit the data during the hard mini-outburst activity.
The broadband spectral energy distributions until the rebrightening are
consistent with the irradiated outer accretion disc model. However, both the
SEDs produced for the peak of rebrightening and close to the peak of
mini-outburst provided good fits only with an additional power-law component in
the UV/Optical/NIR frequency ranges which is often interpreted with a jet
origin.
