I found an image of the Lumin U2 Mini here (link) and it appears the compute module is hidden by the heatsink so it's not possible to know what the streamer is using unless you contact Lumin or remove the heatsink itself. If I had to make an educated guess, though, the form factor suggests it could be a Rockchip based system on module (link) or something else made for embedded computing. If that's the case, the streamer uses an ARM-based CPU (commonly found in smartphones, tablets and small board computers like the Raspberry Pi) and runs some form of Linux as the OS.
If you want to know more, it may be worth your while to post to a DIY audio forum and post some close-up pictures there or send your questions to Lumin directly.