First, it's a bad idea to keep your active library on a cloud or a network drive. You could mirror a copy to your Dropbox, using FreeFileSync or the like, but keep your active library on a local drive. Else your library can easily get corrupted.
Calibre Companion is abandoned, so I'm not sure you can get it to work. AFAIK your library must be in the Dropbox root folder, not in any subfolders, and you must use Gmail for your e-mail address. Beyond that, I have no idea, as I haven't used CC for years.
As to DRM, no, no plugins come with Calibre for removing it. You must install the plugin separately. It can be downloaded from here:
https://github.com/noDRM/DeDRM_tools
We can't give you any other instructions as to how to remove DRM. Ask on the github where you downloaded the plugin. There should be instructions included with the .zip file too.