  We present the results obtained from detailed X-ray timing and spectral
studies of X-ray pulsar Swift J0243.6+6124 during its giant and normal X-ray
outbursts between 2017 and 2023 observed by the Neutron star Interior
Composition Explorer (NICER). We focused on the timing analysis of the normal
outbursts. A distinct break is found in the power density spectra of the
source. The corresponding break frequency and slope of power-laws around the
break vary with luminosity, indicating the change in accretion dynamics with
mass accretion rate. Interestingly, we detected quasi-periodic oscillations
within a specific luminosity range, providing further insights into the
underlying physical processes. We also studied the neutron star spin period
evolution and a luminosity variation in pulse profile during the recent 2023
outburst. The spectral analysis was conducted comprehensively for the giant and
all other normal outbursts. We identified a double transition at luminosities
of $\approx$7.5$\times$10$^{37}$ and 2.1$\times$10$^{38}$ erg s$^{-1}$ in the
evolution of continuum parameters like photon index and cutoff energy with
luminosity. This indicates three distinct accretion modes experienced by the
source mainly during the giant X-ray outburst. A soft blackbody component with
a temperature of 0.08-0.7 keV is also detected in spectra. The observed
temperature undergoes a discontinuous transition when the pulsar evolves from a
sub- to super-Eddington state. Notably, in addition to an evolving 6-7 keV iron
line complex, a 1 keV emission line was observed during the super-Eddington
state of the source, implying the X-ray reflection from the accretion disc or
outflow material.
