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You’re also not trying to get a full CICD pipeline complete with unit/integration tests, crypto signatures, ability to flip features on and off with a click , monitoring of the cicd pipeline, scaled so 1000s of engineers can work at once etc

As a user said below, you wouldn't need cross platform support, incremental builds, tests to run, etc.

Just getting the code to compile, link, baking in assets, etc. For a single architecture is a much more reasonable goal.

I'd imagine it'd be on the scale of 1-3months for an engineer to get working full time, but large error bars around this figure

Plus it already has been done before with Windows XP, without even any documentation and there is a guide on the internet on how to build Windows Server 2003.

This was done (if I remember right) when governments and big customers had access to the Windows source-code.

Microsoft uses Azure DevOps for products as large as Visual Studio.