How to post: tap Download video, then Copy caption. Open TikTok, tap +, then Upload, pick the video, add a sound, paste the caption and post.
iPhone: after tapping Download, tap the blue ⬇ arrow in Safari’s address bar, open the video, tap Share (□↑), then Save Video, so it goes into Photos where TikTok can find it. Android: the video saves to Downloads, and TikTok’s Upload screen can see it.