The Earth’s interior layers -comprising the solid inner core, the fluid outer core, and the mantle- possess their own dynamical properties. These layers interact with one another, creating complex signatures in observations at the Earth’s surface. These signatures are often challenging to interpret, particularly due to the difficulty of distinguishing them from surface processes. In this work, we present a partial review of recent findings that highlight the interactions between the inner core, outer core, and mantle, as revealed by sub-decadal variations in Earth’s orientation parameters and time-variable gravity. We focus primarily on two key interactions: (1) those between the fluid outer core and the mantle, and (2) those between the inner core and the mantle. These interactions influence geodetic observables such as Earth’s length of day and gravity field. Conversely, we also show how geodetic observations can be used to constrain the coupling mechanisms between the core (both inner and outer) and the mantle