In the study of iron-based superconductors, the intrinsic spin excitation spectrum serves as a crucial experimental foundation for understanding the interplay among magnetic correlations, electronic nematicity, and the microscopic mechanism of superconductivity. As multi-orbital Hund's metals, these systems exhibit pronounced orbital-selective electronic correlations driven by the interplay of Hund's coupling and kinetic frustration. In recent years, inelastic neutron scattering (INS) experiments on uniaxially detwinned single crystals have successfully disentangled the intrinsic anisotropic spin dynamics from twinning effects, yielding significant progress. This review focuses on three representative systems: BaFe_2As_2, NaFeAs, and FeSe. At low energies, experiments reveal that spin fluctuations and the spin resonance peak originating from interband scattering exhibit a robust twofold (C_2) rotational symmetry, appearing exclusively at Q_1 = (1,0) and remaining absent at Q_2 = (0,1). This C_2 symmetry provides direct experimental evidence that superconducting pairing is intimately tied to d_yz orbital-selective spin fluctuations. Furthermore, these nematic spin correlations are shown to persist to temperatures well above the structural and magnetic transitions. At high energies, measurements of the full magnetic excitation spectra uncover an energy-dependent orbital hierarchical structure. While low-energy excitations are dominated by the d_yz intra-orbital channel, higher-energy spin waves progressively acquire d_xy orbital character. This transition manifests as Néel-type excitations near (\pm1,\pm1) with C_4 symmetry and distinct, anisotropic damping behaviors. Supported by RPA and DFT+DMFT theoretical frameworks, these findings demonstrate that magnetic excitations in iron-based superconductors are not simple spin flips, but rather composite, multi-orbital cooperative modes. We conclude by discussing the ongoing challenges in theoretically capturing the full dispersion of Néel-type excitations, highlighting the need for further research to fully establish the universal link between intrinsic spin correlations and superconductivity.