Though today much of the Microsoft.Extensions family is considered BCL by most users, especially in the way it is documented as "Core functionality" (the "Generic Host" model uses a lot of Microsoft.Extensions, up front in .NET documentation today) and also in the way that the majority of it (especially in this case of Microsoft.Extensions.Logging) is source controlled side-by-side with most of the BCL, including almost all of System.**: https://github.com/dotnet/runtime/tree/main/src/libraries
(There are some of Microsoft.Extensions that exist outside of dotnet/runtime in dotnet/extensions: https://github.com/dotnet/extensions/tree/main/src/Libraries)