TL;DR: On Almost Any Service You Want
SmartVideo can ingest videos from virtually any service that allows you to make your video file publicly accessible.
For the full provider-by-provider guide, see Where to host your source files. Setup tutorials for Google Drive, Dropbox, Amazon S3, Wasabi, DigitalOcean Spaces, and more are all there. Google Drive share links import files under 100 MB as your original file; larger files import from Drive’s playback copy, so for those, upload the original to the SmartVideo Library (if your plan includes it) or use Dropbox, Amazon S3, or Wasabi.
It might go against conventional wisdom, but if you’re using WordPress and your host provides enough storage, you can keep videos in your Media Library and let SmartVideo handle delivery - no need to upload to YouTube or Vimeo.