Where Are APT Package Sources Stored?
APT reads its package sources from sources.list plus a drop-in directory, which is where every PPA and third party repo lands.
Last updated
Two locations define where Debian and Ubuntu fetch software. The classic sources.list holds the main archive lines, and sources.list.d holds one file per added repository. PPAs and vendor repos always land in the drop-in directory.
Most install instructions append there without explaining the layout, which is how systems collect dead repos over the years. Listing the directory shows every source at a glance, and each file is small enough to read before trusting it.
Where Linux stores this, by platform
/etc/apt/sources.list
Main archive lines live here. Third party additions belong in sources.list.d as separate files, never appended to this one.
/etc/apt/sources.list.d/
One .list file per extra repo or PPA. Removing a repo means removing its file here, then running apt update.
Frequently asked questions
I added a repo and nothing changed. Why?
Run sudo apt update after any edit so the package lists rebuild. Skipping it leaves apt offering stale versions with no error pointing at the cause. Follow with apt upgrade to actually install anything.
How do I remove a PPA cleanly?
Yes. Delete or comment its .list file in sources.list.d, run apt update, and optionally remove its key from trusted.gpg.d. The installed packages stay until you remove them separately.
Notice an outdated path? Let us know.