/usr/share/applications/screensavers/substrate.desktop is in xscreensaver-data-extra 5.15-2ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | [Desktop Entry]
Name=Substrate
Exec=/usr/lib/xscreensaver/substrate -root
TryExec=/usr/lib/xscreensaver/substrate
Comment=Crystalline lines grow on a computational substrate. A simple perpendicular growth rule creates intricate city-like structures. Written by J. Tarbell and Mike Kershaw.
StartupNotify=false
Terminal=false
Type=Application
Categories=Screensaver;
OnlyShowIn=GNOME;
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