When syncing with Dropbox, Joplin creates a sub-directory in Dropbox, in
/Apps/Joplin and reads/writes the notes and notebooks in it. The application does not have access to anything outside this directory.
In the desktop application or mobile application, select "Dropbox" as the synchronisation target in the Configuration screen (it is selected by default). Then, to initiate the synchronisation process, click on the "Synchronise" button in the sidebar and follow the instructions.
In the terminal application, to initiate the synchronisation process, type
:sync. You will be asked to follow a link to authorise the application.