We've actually been discussing this over in this
thread in Mac Accessories.
It basically comes down to the graphics card. With a
Moshi USB-C to DisplayPort Cable you can connect a PC to the XDR but the graphics card needs to support DisplayPort 1.4 and have a high enough link speed to send the signal that supports the desired resolution.
My Nvidia 1080 Ti can handle 5120x2880 (5K) but only at 6 bits per pixel because its link speed is limited to 5.4 Gbps.
Someone else in the thread's Nvidia 2080 Ti is fine at the XDR's native resolution of 6016 x 3384 (6K) at up to 10 bits per pixel.
It is similar to the range of Macs that support various resolutions on the XDR. The more modern and powerful the graphics card, the higher the resolutions it will support.