  We present a comprehensive multi-wavelength spectral analysis of the black
hole X-ray binary MAXI J1820+070 during its 2018 outburst, utilizing AstroSat
far UV, soft and hard X-ray data, along with (quasi-)simultaneous optical and
X-ray data from Las Cumbres Observatory and NICER, respectively. In the soft
state, we detect soft X-ray and UV/optical excess components over and above the
intrinsic accretion disk emission ($kT_{\rm in}\sim 0.58$ keV) and a steep
X-ray power-law component. The soft X-ray excess is consistent with a
high-temperature blackbody ($kT\sim 0.79$ keV), while the UV/optical excess is
described by UV emission lines and two low-temperature blackbody components
($kT\sim 3.87$ eV and $\sim 0.75$ eV). Employing continuum spectral fitting, we
determine the black hole spin parameter ($a=0.77\pm0.21$), using the jet
inclination angle of $64^{\circ}\pm5^{\circ}$ and a mass spanning
$5-10M_{\odot}$. In the hard state, we observe a significantly enhanced
optical/UV excess component, indicating a stronger reprocessed emission in the
outer disk. Broad-band X-ray spectroscopy in the hard state reveals a
two-component corona, each associated with its reflection component, in
addition to the disk emission ($kT_{\rm in}\sim 0.19$ keV). The softer coronal
component dominates the bolometric X-ray luminosity and produces broader
relativistic reflection features, while the harder component gets reflected far
from the inner disk, yielding narrow reflection features. Furthermore, our
analysis in the hard state suggests a substantial truncation of the inner disk
($\gtrsim 51$ gravitational radii) and a high disk density ($\sim 10^{20}\ \rm
cm^{-3}$).
