If you buy something from Tiktok, you presumably visit the merchant's website, which almost surely will have chosen to have a tracking pixel that sends data to FB (Instagram). You can read a bit about how tracking pixels work here: https://jvns.ca/blog/how-tracking-pixels-work/

In this case it's not Tiktok and Instagram that are sharing data with each other, but the product website that is choosing to share data with both of them.