No. Xbox 360 demos were exclusively distributed via Xbox Live Marketplace. With the Xbox 360 Store closure (July 2024), even that official avenue is gone for purchases, though previously "purchased" (free) demos can still be re-downloaded from Download History on the console or via account.xbox.com.
.xex (executable) is signed by Microsoft for a specific Title ID. Unsigned code won't run on retail 360 (requires JTAG/RGH)..xvd / .xvc containers with license metadata tied to Xbox Live account..xex files. Communities (XeBuild, DashLaunch) have archived demo .xex files. Requires hardware mod (soldering) + legal risk..xex files. Legal if you dump your own downloaded demo.Demos do not work via backward compatibility. Only full games on the BC list. You cannot play 360 demos on Series X|S.
If you never "purchased" the free demo back when the store was open, you're out of luck on original hardware. Watch footage or use Xenia (PC) with a legally dumped demo.