The blue tab sata data cable for the upper right hdd cage is by default included. You don’t need to buy a sata data cable if the sata ssd is the only sata storage device you plan to have in pc.  A 2.5 to 3.5 adapter is needed (not included) for 2.5” sata ssd.
My existing PC has a separate physical SSD for data. It's a 2.5-inch, SATA III drive. If I order an 8950 with a single M.2 NVMe drive, will I be able to fit my existing SSD into the chassis as my data drive? (I'm familiar with the settings in Windows 11 that I will have to change to make the SSD the default data drive.) If so, do you think this is a good idea? I am aware of this discussion: XPS 8950, adding two 2.5 SDDs in addition to 3.5 HDD, however it's not clear to me from that discussion whether I would need to buy a data cable and/or a 2.5-to-3.5 caddy. Eventually (after the warranty is over) I'll buy an aftermarket M.2 NVMe for my data drive. I'm just trying to avoid spending $50 on Dell's HDD (as the second drive) until then, if possible. As always, thanks for everyone's advice.
XPS 8950, can I add my own Data drive?