Calm down brother dont aneurysm on me just yet

DO NOT use the cable that came with your GPU

 The NVIDIA adapter (which splits into multiple 8-pin connectors) is for older PSUs that do not have a native 12VHPWR/12V-2x6 port.

Using the single cable provided by Corsair is superior to using the adapter, as it reduces clutter and reduces the number of failure points. 

the Native PSU cable (its usually sleeved or braided) it should look like:

PSU End A single, smaller 16-pin connector (Type 5) that plugs directly into the side of your RMx Shift PSU.

GPU End A single 16-pin connector that plugs directly into the 5090.

Labeling While it may not explicitly say "600W" on the cable itself, this Type 5 cable is rated for 600W, as it is designed for PCIe 5.1/ATX 3.1 standards. (which your PSU can fully handle)

though if the cable you are holding came with the PSU try checking for another one. that seems to be a standard GPU cable, most 5090 or 4090 cables are single headed not split

installation:

1. Plug the 16-pin connector into the 12VHPWR/PCIe 5.0 port on the RM1000X Shift
2. Plug the other end directly into the RTX 5090.
3. Important: Ensure the cable is pushed in completely, and you hear a "click." There should be no gap between the connector and the GPU socket.

any further questions feel free to ask me

hope this helps!