X-ray binaries (XRBs) consist of a compact, degenerate stellar remnant in a gravitationally bound system with a stellar companion. The X-ray emission is mainly due to the mass transfer from the stellar companion onto the compact object, such as a neutron star (NS) and black hole (BH), through the accretion process. Studying these objects will be a crucial step towards understanding two separate phenomena: accretion onto compact objects and testing general relativity in the strong-field regime. In this talk, I will discuss two classes of X-ray binaries: Galactic black hole X-ray binaries and ultra-luminous X-ray sources. Black hole X-ray binaries (BHXRBs) spend most of the time in the quiescence phase and occasionally exhibit outbursts. During the outburst evolution, these systems exhibit different spectral and fast-timing properties, which are important to understanding the system and its geometry. Considering this, we used the energy-dependent variability characteristics such as RMS and the time lag of the BHXRBS and modelled them using the spectro-timing model. The modelling provided information regarding the origin of the variability and the mechanism responsible for the variability. I will highlight the potential of such techniques to unravel the radiative process responsible for variability using high-quality spectral and temporal data from AstroSat and NICER. In the second part of the talk, I will discuss the enigmatic sources, Ultra-luminous X-ray sources (ULXs), which are off-nuclear, bright, X-ray point sources with X-ray luminosities above 10^39 erg/s. Since the observed luminosity of ULXs exceeds the Eddington luminosity for a stellar-mass black hole. Thus, ULXs will be an ideal class of sources to explore the super-Eddington accretion regime. Using multi-mission data, we studied the broadband X-ray spectral and temporal properties of NGC 55 ULX-1. Our study revealed spectral parameter correlations, such as anticorrelation between the characteristic radius and temperature of the blackbody component, positive correlation between the blackbody and inner disc temperatures, and marginal evidence of X-ray dips at different intensity levels. The observed spectral and temporal properties suggest a supercritical radiatively driven outflow from the source.