Mass curves spacetime, and spacetime tells mass how to move.
The Einstein field equations encode this precisely. The Einstein tensor on the left (built from the Ricci tensor and scalar) measures how spacetime geometry deviates from flatness.
In the standard form (natural units), the equation reads R_αβ − ½ R g_αβ + Λ g_αβ = 8π T_αβ. Geometry responds to matter; conservation of energy-momentum follows automatically from the Bianchi identities.
Curvature is not a force; it is the geometry itself.