eMMC with Pine64 512K

[Luke]

I assume you're asking about the 512mb PineA64 board. For starters, that will not run Android. Secondly, it does not have an eMMC socket.

As for your other questions, eMMC is used as a replacement for a SD card (for boot and rootfs); you could theoretically boot from SD and mount it as storage too - but I cannot see a usecase scenario for that.

Using eMMC instead of SD results in much better performance in android.