Where is my MMC device?

You're working in a container, so you do not have access to the real /dev filesystem – the only devices available to you are those that were created or linked by the container manager.

(As you can see from 'findmnt', /dev is a "tmpfs" – not a "devtmpfs" – so its contents are not managed by the kernel; in this case systemd-nspawn has provided a few standard device nodes, and bound a few specific ones from the host's /dev.)

The 'lsblk' tool can see the devices through the /sys filesystem (which the container provides a read-only but real version of), but the names it shows are computed from information available through /sys, not from directly looking in /dev.