Just click on upload videos below upload images make a saint account if you don’t have one and if you do just log in. Press on drop file here to upload and pick whatever video you want to share after name the album whatever you want and just click on the album and upload it.
(You could also just use pixeldrain which is much better imo)
For iPhone you can download (1DM Browser) then login to your only fans I’d and play the video then the download icon will show to download the video.
or
you can try aloha browser, it has an option to download the videos you play