Generally speaking, the backplane would have the connectors for both power and SATA/SAS. The backplane would then connect to either PCIe HBA card or to the motherboard itself.
In this post there was a picture of the HL-15 with just the backplane installed via @Glitch3dPenguin who only bought the case and backplane (as he shared in the post).
As the Z690 does not have a builtin HBA, do you have a separate PCIe card (for your HBA)?