It was the Microsoft’s official Python extension, as far as I recall. It was possible to use some other extension for typechecking but there were some other issues with it. (Now everything works perfectly in Neovim, and my setup respects to only use typing stubs I specify in the project.)
The Microsoft official Python extension uses Pylance, which is a closed-source extension of Pyright with additional features such as built-in type stubs. This is probably what you saw.
If they were truly built-in I would not mind, but I found a bunch of third-party stubs in a dependency tree of the virtual Python environment VSC created (and ran obviously unsandboxed). What’s worse is that stubs, in addition to not being specified by me, were often pulled at the wrong version compared to the package I was using, leading to typing mismatches and runtime errors.
