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Bing is huge both in codesize and technologies used, but most of it is a flavor of Windows Server 2016 (soon 2019) + http.sys + C# + Razor + TypeScript for Frontend. C#/C++ for middle and lower tiers.

https://github.com/dotnet-architecture/eShopOnContainers

While it's not Bing, this is the canonical example I have seen in response to a lot of "What is at least a very good way to do x in .NET Core?"

Most of the stack is custom.