The ty repo contains the ruff repo[1] as a submodule, where the remainder of the code is. It is indeed open source, the layout is just indirect at the moment because of code-sharing between the tools.

[1]: https://github.com/astral-sh/ruff

This is also documented at https://github.com/astral-sh/ty?tab=readme-ov-file#contribut... and https://github.com/astral-sh/ty/blob/main/CONTRIBUTING.md#re...