It's only okay when MY favorite country does it.

More like, tiktok is our specific version

Douyin is what China has. That came first, then the parent company developed tiktok for release outside of china.
Then they bought Musical.ly, merged it with tiktok, and that became the modern version of the app

It's not the same platform. Douyin has a different, separate database of content. Even though they share the same idea of how the app should function, they are disjoint social media platforms.