TikTok
Regular video links and short links (vm.tiktok.com, vt.tiktok.com) both work; short links are resolved automatically. TikTok photo slideshows are supported in photo flows, but a video-only template will tell you clearly if you hand it a slideshow. Downloads prefer the watermark-free version of the video where available.
X / Twitter
Paste any tweet link (x.com, twitter.com, or t.co short links). Video tweets give you the video, photo tweets the photos (multi-photo tweets included), and the tweet text is used for captions. A text-only tweet has no media to download, but it's a perfect source for a Tweet Rebrand template.
Reddit videos come with their audio merged in, and image posts and galleries are supported. A text-only Reddit post has no media, and Socialix says so instead of guessing.