Connectivity is a bit limited because everything is directly connected to the CPU. There is no northbridge/southbridge. There’s a board diagram on the Wiki for the Talos II:
I imagine the Blackbird diagram will be pretty similar, minus the second socket and with the SATA controller in place of the Talos II’s SAS. No idea where the onboard audio will connect, though.
Each POWER9 CPU (Sforza specifically) has 48 PCIe lanes, but only supports 6 PCIe endpoints, which is why you see some lanes left unused. Maybe this is also why arbitrary PCIe lane bifurcation is not supported.