SGWB from extragalactic DWDs for the various types of DWDs and for different metallicities of stellar progenitors. In each sub-figure, the solid colored line shows the energy density spectrum of the total SGWB from this type of DWD while the dashed, dash-dotted, and dotted lines show the contribution of DWDs formed by the evolution of stellar progenitors around metallicity Z_⊙, 0.1 × Z_⊙, and 0.01 × Z_⊙ respectively. Colored lines indicate the contributions from different DWD types. The plot highlights the overall SGWB with different line styles for each type of DWD binary: CO-CO binaries are shown in blue, CO-He binaries in orange, He-He binaries in green, ONe-CO binaries in red, ONe-He binaries in brown, and ONe-ONe binaries in olive.