Normally, when you close Cydia, it deletes the installation deb files for the tweaks you recently installed.
DebKeeper disables this function.
If you enable the "Keep" option, you can find the files in /var/mobile/Library/ Caches/com.saurik.Cydia/archives/.
If you enable the "Move" option, you can find the files in /var/mobile/Downloads/Cydia/.
If using the "Keep" option, you should manually move the files to another location for safe keeping, especially since DebKeeper does not work in SafeMode so, if you leave your files there and use Cydia in SafeMode, they will be deleted.