There are two main ways to upload files from Google Drive: direct upload and using linked files.
If you go to the Project menu, click Files..., and then Upload from... Google Drive, you can connect your Google Drive account with Overleaf and upload directly from Drive. This is useful if you are on a tablet or phone, and your files are stored on Drive.
If you make the file shareable via link on Google Drive, you will get a link of following form:
On Overleaf, you can then use the FILE_ID to link a file in your project to that URL. To do this, go to the Project menu, click Files..., and then Upload from... URL and enter
where FILE_ID comes from the sharing link above.
Now, if you make a change to the file on Drive, you can pull the new version into your Overleaf project by clicking the refresh link on the linked file.
This works particularly well if you have a Google Drive folder on your computer. Then you can save a file, e.g. an export from a bibliography manager, to your Google Drive and refresh it with one click in Overleaf. You can also use the 'Refresh linked files option to refresh all of your linked files, or the linked files in a given folder.
Note that if the file is not set to allow access to anyone with the link, or if you just use the sharing link directly without going through the process above, you may find that the PDF or image file that you upload from Google Drive by URL is not valid. This is because Overleaf will save the Google Drive web viewer page us rather the file itself. If you go through the process above, you should be able to link to the file directly.