I think they use the SIM card country ID and also the ad audience categories to suggest content.
For example swapping the SIM card with a fresh installation of TikTok shows different "local" videos. Even if the public IP address is unchanged like when using WiFi instead of Data.
I've also think they use ad categories to suggest videos. Using a fresh TikTok installation and then spending some time navigating a subreddit of a topic, would likely show videos of that topic the next time you open TikTok.
For example swapping the SIM card with a fresh installation of TikTok shows different "local" videos. Even if the public IP address is unchanged like when using WiFi instead of Data.
I've also think they use ad categories to suggest videos. Using a fresh TikTok installation and then spending some time navigating a subreddit of a topic, would likely show videos of that topic the next time you open TikTok.