Moon
you should burn the:
Moon batocera-5.20-x86_64-20190217.img.gz
the boot.tar.xz is for down- or upgrading manually (you would then just have to extract this archive to your pen drive)
for flashing the *.img.gz files i recommend to use etcher. Just choose the file (without needing to extract before) within etcher, then select your pen/flash drive, where you want it to be installed on and press start/flash.