I believe they use a different backend/storage. Different content shows up depending if you are searching from Tiktok or Douyin. Perhaps someone from Bytedance is curating select content from Tiktok for Douyin.

Either way, if I put a video on Tiktok it does not mean Chinese users on Douyin will see it.

https://chinasocialmedia.net/tiktok-and-douyin-are-not-the-same/