If you're uploading videos to Google Classroom to turn in an assignment, you need to first upload the video to Google Drive, and then use a link to turn in the video. Follow the steps below:
Upload the video to Google Drive. Assuming you're using a mobile device to record the video, use the Google Drive app to upload the video to Google Drive.
Find your video on Google Drive, and use the SHARABLE LINK / COPY LINK option to copy the link URL.
Paste the URL as a LINK when you are ready to upload your work (see image).