If your iPhone/iPad is iOS13 or higher, you can use the Safari browser to download Tiktok Video.
If not (iOS is under 13), you can't normally download any videos Mp4 TikTok, music (Mp3), or movies to your iPhone just like you can do it with Android phones. But there are some methods to deal with it, please follow this introduction How to download TikTok videos with iPhone (IOS).