It is started via dbus, which is configured in /usr/share/dbus-1/services/org.freedesktop.secrets.service
to
[D-BUS Service]
Name=org.freedesktop.secrets
Exec=/usr/bin/gnome-keyring-daemon --start --foreground --components=secrets
To use keepassxc as the secret service provider, add into ~/.local/share/dbus-1/services/org.freedesktop.secrets.service:
[D-BUS Service]
Name=org.freedesktop.secrets
Exec=/usr/bin/keepassxc