Pixels largely use open source code for the parts specific to Pixels (TEE, secure core, secure element, boot chain) but unfortunately they don't publish all the Pixel specific changes yet. They committed to publishing the firmware and hardware sources for the Titan M but provided no timeline and clearly didn't mean they would publish the sources for the current revisions but rather future ones. There's the OpenTitan project which they seem to use as a base since the Pixel 6 but that isn't what they committed to doing. We're still waiting for this and hoping they do it soon so that we can help audit this and report better bugs when we run into issues.