The best way to understand the logical structure of a relational database in Access is to examine the relationships window. Unfortunately that in Northwind has not been set out to make the model self evident, so you'd need to move things around a lot first.
For a simple example you might like to take a look at Relationships.zip in my public databases folder at:
https://onedrive.live.com/?cid=44CC60D7FEA42912&id=44CC60D7FEA42912!169
If you have trouble downloading a specific file, clicking on the 'Download' button at the top of the page while no files are selected should download a zip file of all files in the folder, from which you should then be able to unpack the relevant file.
This simple little demo illustrates how the relationships are built up across the database to achieve the final model, which is: