Your Trilium data is stored in a SQLite database
which contains all notes, tree structure, metadata, and most of the configuration.
The database file is named document.db and is stored in the
application's default Data directory.
When first starting Trilium, it will provide a set of notes to showcase various features of the application.
For more information see Demo Notes.
Trilium provides a lot of flexibility, and with it, opportunities for advanced users to tweak it. If you need to explore or modify the database directly, you can use a tool such as SQLite Browser to work directly on the database file.
See Manually altering the database for more information.
If you are experimenting with Trilium and want to return it to its original state, you can do that by deleting the current database. When you restart the application, it will generate a new database containing the original demo notes.
To delete the database, simply go to the data directory and
delete the document.db file (and any other files starting with document.db).
If you do not need to preserve any configurations that might be stored
in the config.ini file, you can just delete all of the data directory's contents
to fully restore the application to its original state. You can also review
the configuration file to provide
all config.ini values as environment variables instead.