iCloud Photos only store videos taken with iOS device?

Upon logging into icloud.com on my PC and navigating to Photos, I can see all videos taken with my iOS device. However, uploading photos (in .jpg format only), is not an issue to convert from other formats, it's the videos that are the problem. I'm wondering why you can only download videos from iCloud Photos, and not upload. I understand that uploading videos to iCloud drive is possible, but not for iCloud photos? Thx!