Can videos be uploaded from one video sharing site directly to other video sharing sites?

These video services are direct competitors. I don't believe they'll help you do this or make it easy for creators to do this.

What I do see is YouTube creators, when certain videos will likely be taken down due to possible content strikes from overzealous IP holders despite the videos in question are 1000% fair use, upload them to Twitter or Vimeo, etc... and drop a YouTube channel post with said offsite link to the other service where the video is hosted.

I suspect the differing proprietary file formats each service uses (esp thinking of YouTube) is literally developed to prevent or better said ... make pretty difficult for the YouTube creator to spread content over several platforms.

On the other hand, don't spread your content too thin over several hosting sites. This would be ultimately detrimental to creating a fanbase for your work because it will be difficult to get past a tipping point of viewers/channel members on a video service and thus get noticed/recognized by that channels algorithm and thus perhaps earn actual money from your content.

I want to upload my videos on as many online video platforms as I can.

Literally, no one is stopping you. I believe you have to upload them individually to their respective sites.

What I'm referring to is a shortcut method of uploading videos to multiple video platforms. I'm going to try using my shortcut method of uploading videos to multiple video platforms and see if that works. It would be easier to upload a video to one site and then upload it from that site to other sites.

I'm aware these sites are competitors but I want to try this method anyway. It wouldn't hurt.