In a simplistic way, darktable has two main views to manipulate your images (there are more but lets keep it simple.
When you start darktable you are in the "lightable view and the images are shown as thumbnails. This is where you import/export browse, catalog, search. move. copy and generally look after your images.
When you double click an image you are taken to the "darktable" view where you edit the image using the edit modules. When you finish editing you go back to "lightable" view
To answer your question, when in lightable select the import tab. Using the box that opens, can you navigate to your external drive and see the folders and images? If the answer is yes, then you can leave them on your external drive and import into darktable.
Darktable will make a reference in its own database to where they exist on your external drive and it will work just fine. If you disconnect the drive and start darktable, darktable will still show all of the folders in the external drive but thumbnails will be replaced withy skull icons. This means that darktable cannot fines the images. Plug the drive back in, restart darktable and it will all be working again
There are a lot of modules in darktable and it will take a while to get to grips. Do not be frightened to try anything, nothing you do (apart from selecting the trash to delete an image) will harm you images, it is all non destructive editing. You can make the most bizarre edits, or test something, or just play to see what happens, and when you finish, you can revert the image back to how you started if you want another go