Your engineers run the app after we leave. That only works if the last month is designed around it.
We pair with whoever inherits the codebase for two weeks while they run a release themselves. Architecture notes and runbooks are written during the build, not compiled at the end, and every runbook has been executed by someone on your side at least once.
The measure of a good handover is simple: six months later, has the app shipped again? If nobody has touched it, we did the handover badly.