Here is an alternative way to download and at the same time directly write to a USB storage device without the need to store the compressed file to local storage first, hence saving both space and time:
root@host:/home/me# wget "https://download.freebsd.org/ftp/releases/amd64/amd64/ISO-IMAGES/12.1/FreeBSD-12.1-RELEASE-amd64-dvd1.iso" -O /dev/sdX
Maybe it could be mentioned on the website.