How does Google evaluate TPM candidates in a System Design interview?

The system design interview for a Technical Program Manager (TPM) at Google is less about architectural depth (i.e. knowing in details the features of latest and greatest platforms) and more about having a structured, methodical approach to problem-solving and technical program management. At the end of the day, as a TPM, you'd be interacting with PMs and SWEs, and should understand design decisions and trade offs, and have a reasonably educated POV. The expectations change with seniority.