There are 2 places to file a Google Doc into a folder (besides Google Drive). The first is the folder icon right next to the document title.  You will also find the folder icon under the File menu listed as “Move to folder…”

Multiple Folders

When you choose to put a document into a folder you can actually choose to put the document into MULTIPLE folders. Hold down the command key (control on a PC) and click on multiple destination folders when organizing your document.

Notice the multiple checkmarks noting the document is in multiple folders.