  We report a broadband investigation of the Z-type neutron star (NS) low mass
X-ray binary (LMXB) GX 349+2 using AstroSat and NICER. AstroSat observed the
source exhibiting large scale variability in its normal branch (NB) /flaring
branch (FB) vertex and flaring branch (FB) and a moderate evolution during
NICER observations. The power spectra exhibit very low-frequency noise (VLFN)
and low-frequency noise (LFN)/flaring branch noise (FBN), described by a power
law and an evolving Lorentzian. We investigate the energy dependence of
variability components and their correlation with the spectral state to probe
their origin. The joint spectra of GX 349+2 are modeled by two thermal and one
non-thermal component. The source moves along the Z track, with the increasing
accretion rate, further heating of the NS boundary layer, and increasing
temperature/radius of the brightened hotspot at the disc-boundary layer
interface/NS surface. A power law well represents the hard non-thermal coronal
emission. As predicted by the gravitational redshift, we find a correlation
between the line energy detected in NICER spectra and the inner disc radius
with the Spearman rank correlation coefficient of 1. Using this correlation, we
demonstrate the potential of a method to constrain the accreting compact object
properties, including evolving continuum and line spectroscopy. We report the
first detection of hard lag providing evidence of the VLFN originating from the
accretion disc in NS LMXBs, representing fluctuation of propagation through the
disc.
