  We present a comprehensive spectral and temporal study of the black hole
X-ray transient MAXI J1820+070 during its outbursts in 2018 using Swift/XRT,
NICER, NuSTAR and AstroSat observations. The Swift/XRT and NICER spectral study
shows a plateau in the light curve with spectral softening (hardness changes
from $\sim$ $2.5$ to $2$) followed by a gradual decline without spectral
softening during the first outburst. Also, spectral modelling suggests that the
first outburst is in the low/hard state throughout with a truncated disk
whereas the thermal disk emission dominates during the second outburst. During
the entire outburst, strong reflection signature (reflection fraction varies
between $\sim$ $0.38 - 3.8$) is observed in the simultaneous wideband
(NICER-NuSTAR, XRT-NuSTAR, AstroSat) data due to the presence of a dynamically
evolving corona. The NICER timing analysis shows Quasi-periodic Oscillation
(QPO) signatures and the characteristic frequency increases (decreases) in the
plateau (decline) phase with time during the first outburst. We understand that
the reduction of the electron cooling timescale in the corona due to spectral
softening and the resonance oscillation with the local dynamical timescale may
explain the above behavior of the source during the outburst. Also, we propose
a possible scenario of outburst triggering and the associated accretion
geometry of the source.
