The 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquakes are the latest major crustal earthquakes in southern California. Geological surveys and satellite observations provide important constraints on the surface ruptures of faulting. However, the subsurface fault geometries, which are important for understanding earthquake initiation, propagation, and termination are not well resolved. In this study, we conduct multi-scale earthquake source studies (from aftershock point source moment tensor inversion to mainshock multi-fault finite fault inversion) using a 3D velocity model. The major findings are (1) Pre-/post-mainshock stress rotations, while different sense and magnitude of stress rotations observed along different segments. This can be largely explained by the varying trends of the fault segments in relation to the regional stress field; joint analysis of different segments suggests that the ratio of mainshock stress drop to the background stress to be 0.5-0.9, indicating a large fraction of the initial stress has been released co-seismically; (2) Complex flower structures, splay, and antithetic faults at shallow depth, while a simple large through-going fault exists at >5 km depth. We suggest that the complex fault geometries partially controlled the rupture patterns of the 2019 Ridgecrest sequence.