Fedora 43 forgot to make the following a default package in order for thumbnails to keep working. Need to manually install it.
dnf install glycin-thumbnailer
The freedesktop.org thumbnail specification puts thumbnails at:
~/.cache/thumbnails/<size>/
The filename is a MD5 hash of the URI.
Delete “thumbnails” to force regeneration. For example, a setting could be changed, then close Nautilus, and then delete the thumbnail cache. Reopen Nautilus to see the results immediately.
Turn off and on with:
gsettings set org.gnome.desktop.thumbnailers disable-all false
Set overrides in: ~/.local/share/thumbnailers
Deleting this directory causes fallback to the system defaults. Most users don’t have overrides.