Darktable saves edits in sidecar files. Sidecar files have the file extension .xmp and are a list of instructions to reproduce the history of edits.

Because the edits are stored as instructions in the sidecar file, no changes need to be made to the original image whether it is RAW, JPG, TIF, DNG, or something else. That's why Darktable is considered non-destructive (another term for "non-destructive editing" is non-linear editing).

Darktable stores meta-data about the source and the edits in its databases. The databases are managed using SQLite3. On Ubuntu, they are located in ~/.config/darktable. The file library.db contains metadata about pictures and their edits. There is also data.db containing data for styles and module presets. Both can be explored (and modified) using standard SQLite tools.

Unlike RawTherappee where sidecar files are more or less human readable, Darktable's sidecar files are rather cryptic. They are however, in plain text and can be manipulated using your favorite text tools. It is also possible to create new sidecar files by copying and editing an existing file...this is what the "duplicate" action does within Darktable...it just creates a new sidecar file.