I’m sure there are a few open source developers who use brew or Xcode directly, but most Mac builds happen in a distributed build system wired up to use remote macs. Yes, via Xcode. Not on local machines.

The mac build system is wild. It is still all done remotely with a farm of macs running xcode. You still don't build code for running on macs on your local machine.

You actually do, what Tulsi generates is an XCode project that has shell script build steps that call out to bazel. Bazel underneath the covers will end up calling the clang that comes with XCode.