The build platform's components (compiler, interpreter, build tools) are hardened, isolated and secured, and the first kind of evidence gathered from a build is the build system's own confirmation that the tools and processes ran in an isolated environment. The second kind is a hash of the finished artifact together with the files, libraries and other materials used and all events that occurred during the build.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.