There's fundamental difference between C# standard library and Rust standard library.
C# includes in it's standard library lots of things which may be nice for the developers to have.
Rust tries to include only things which have to be in the standard library, lack of which may fragment Rust community (imagine standard library without
String) or which are not possible to implement in a stable Rust (like
Everything else belongs to the “third-party” crates.
P.S. It would have been really nice for novices if there would have been a nice set of crates which can, together, be considered a replacement for C#/Java/Python python SDK. Set of well-developed crates which solve common needs and are considered “good enough” today. There would be no need to keep everything in that list supported “forever” (like the stuff in standard library) but at least it would be a place where you can go and find out that crate Foo is no longer supported and Bar is it's incompatible replacement. Like “RLS is obsolete, try rust-analyzer instead” (something which makes quite a few total newbies make the wrong choice).