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The light-echo(reverberation mapping) technique is a powerful tool to probe the central regions of active galactic nuclei(AGN), inaccessible to current imaging capabilities. The structure of the broad line region(BLR) and the physical origin of continuum time delays in AGN are not understood. To address these, we have developed a model that combines a standard accretion disk with an optional inner hot flow,a lamp-post irradiation geometry,and a BLR structure shaped by radiation pressure acting on dust. The model is parameterized by the black hole mass,accretion rate,viewing angle,lamp-post height,cloud density,and cloud covering factor. With this framework, we are able to reproduce the broadband emission, the Hβ reverberation delay, and the observed inter-band continuum time delays. By performing simultaneous fitting of the lag-spectrum and the spectral energy distribution, while treating the accretion rate and luminosity distance as free parameters, this approach enables an independent determination of the Hubble constant. Motivated by this, we have initiated a photometric AGN monitoring program, Hubble Constant Constraint through AGN Light curve Observations(HALO) at the Obserwatorium Cerro Murphy in Chile. In this talk, first,I will present the HALO program, focusing on modeling time delays, and discuss its synergy with the LSST survey in addressing the Hubble tension. Second,I will present recent results from BLR reverberation mapping from the Seoul National University AGN Monitoring Project,including an updated BLR size-luminosity relation for improved single-epoch black hole mass estimates. Finally,I will discuss dust reverberation mapping results that constrain the size of the dusty torus in AGN. Hence,by unifying accretion disk,BLR,and dust reverberation modeling with dedicated AGN monitoring,our framework provides an independent route to probe AGN inner structures and constrain cosmological parameters in the LSST era.