There used to be a Python sync server used in the Chromium project that was used to test sync functionality. Someone discovered this and yanked out the relevant code to enable self hosting.

https://github.com/infeeeee/chrome-sync-server

Though its since been replaced by some c++ fake server. https://chromium.googlesource.com/chromium/src.git/+/refs/he...

Looks like it is? https://www.chromium.org/developers/design-documents/sync

And even if not, since the client is OSS, it can be inferred.