On a 1080 × 1920 video, TikTok draws its buttons down the right edge and the caption along the bottom. Open a photo or a video frame, place your on-screen text, and see how much of it ends up under the app.
A landscape picture has to be cropped, padded or both before it fills a phone. Each version below is drawn from the same picture at 1080 × 1920, with how much of the original survives and how much of the screen it fills.
Over the video, the caption is cut after about two lines at the bottom-left, the same strip the safe-zone check above treats as covered. The page wraps your caption at that width and lists each hashtag with its length, so you can see which ones fit before the fold.