This question that comes up a lot because the assignment submission options are kind of oddly named in Canvas. In your assignment’s settings, scroll down to the Submission Type box, and (B) select Online on the dropdown menu’s options, then (B) select the Website URL checkbox.
When students go to submit their assignment, they can just paste the sharing link for their Google Doc into the textbox, or they’ll see a tab for Google Drive, which they can use to search for the folder within their Google account. You can view them if you use the Student View feature.
Check out the instructor guides for assignments on the Canvas website if you’d like to learn more about them.