~/.config$ more user-dirs.dirs
holds the defaults for all the directories created by the system for the user. I use a HDD mounted as /discworld
to keep my SDD clean from user files:
# This file is written by xdg-user-dirs-update
# If you want to change or add directories, just edit the line you're
# interested in. All local changes will be retained on the next run.
# Format is XDG_xxx_DIR="/discworld/yyy", where yyy is a shell-escaped
# homedir-relative path, or XDG_xxx_DIR="/yyy", where /yyy is an
# absolute path. No other format is supported.
#
XDG_DESKTOP_DIR="/discworld/Desktop"
XDG_DOWNLOAD_DIR="/discworld/Downloads"
XDG_TEMPLATES_DIR="/discworld/Templates"
XDG_PUBLICSHARE_DIR="/discworld/Public"
XDG_DOCUMENTS_DIR="/discworld/Documents"
XDG_MUSIC_DIR="/discworld/Music"
XDG_PICTURES_DIR="/discworld/Pictures"
XDG_VIDEOS_DIR="/discworld/Videos"
You can change only Dowloads to the new location and that will be used unless you set a download inside software itself. Like Firefox, Chrome all have a setting for downloads to override the default.