Summary: We derive the generalized Gauss-Codazzi-Mainardi (GCM) equation for a general affine connection with torsion and non-metricity. Moreover, we show that the metric compatibility and torsionless condition of a connection on a manifold are inherited to the connection of its hypersurface. As a physical application to these results, we derive the (3 + 1)-Einstein field equation (EFE) for a special case of metric-affine \(f(\mathcal{R})\)-gravity when \(f(\mathcal{R})=\mathcal{R}\), the metric-affine general relativity (MAGR). Motivated by the concept of geometrodynamics, we introduce additional variables on the hypersurface as a consequence of non-vanishing torsion and non-metricity. With these additional variables, we show that for MAGR, the energy, momentum, and the stress-energy part of the EFE are dynamical, i.e., all of them contain the derivative of a quantity with respect to the time coordinate. For the Levi-Civita connection, one could recover the Hamiltonian and the momentum (diffeomorphism) constraint, and obtain the standard dynamics of GR.
For Part II see [the authors, ibid. 38, No. 22, Article ID 225006, 42 p. (2021; Zbl 1479.83006)].