Moving a Video from Classic Stream to Stream on SharePoint
To transfer a video from Classic Stream to Stream on Sharepoint you must first download the video from Stream.
Download from Classic Stream
Go to the video you wish to transfer. Click on the ellipses ... and select Download video.
(You must have uploaded the video or be Owner)

The video will now download to your Downloads folder.
Upload to Stream on Sharepoint through Microsoft Teams
Go to the Channel in Microsoft Teams that you wish to share the video with. (Note: everybody in the team will be able to view the video)
From the Files tab, select Upload and Files.

Browse to the video and click OK.
Upload to OneDrive
Files stored on OneDrive are not shared with anyone else. You may want to save them to OneDrive for now, then upload to SharePoint at a later date.
Within OneDrive online, click Upload, then Files.

Select the videos you want to upload and click Open. (Note: You can also upload files by opening File Explorer, selecting the files you want to upload, and dragging them to OneDrive.)

Your video will now upload to OneDrive.