These are direct dumps of physical game cartridges, primarily used in PC emulators like Citra or its modern successors.
Before working with DLC archives, your 3DS must be modified. 3ds dlc archive
In North America, half the game’s content was locked behind a "buy the full game" DLC patch. The eShop is gone, so a vanilla cartridge only contains the demo levels. These are direct dumps of physical game cartridges,
Certain games had extensive or high-value DLC that preservationists have prioritized: 3ds dlc archive
If you want to explore the technical side of preservation, let me know:
GodMode9 is a powerful file browser for the 3DS that allows you to dump system files and cartridges. Power off your 3DS.