I installed joplin cli on Windows, it works fine but when I open it , it starts with an empty notebook. I already have the desktop version of Joplin with alot of notes on the same PC. Is there a way to let the cli version access the same notebooks so that I do not end up duplicating data for no reason.
The desktop app and the terminal app aren't necessarily kept in sync, and the content of their databases can differ. The only supported way to have the terminal version and desktop version showing the same files, is to have a sync target that both can access as if you were sharing amongst different computers entirely. (The sync target can just be a local folder on the PC, if there's no other devices synchronising).
Any attempts to work around that limitation will almost certainly lead down a path of pain with potential data loss in the future. People have done it by using symbolic links to trick the terminal app into accessing the desktop apps databases. The result is it usually works ok for a while and then breaks everything a few months down the line with data loss being a legitimate concern.