Valve uses their own tier0 and tier1 libraries in both the Source Engine and Steam, they implement their own standard library like functions and I believe it's where the memory allocator is too. tier1 source is available [0] whilst I don't think tier0 has ever been made public, though it's headers are [1], if you look hard enough you can find the complete source code for Source 2007 though which will include tier0.

[0] - https://github.com/ValveSoftware/source-sdk-2013/tree/master...

[1] - https://github.com/ValveSoftware/source-sdk-2013/tree/master...