I know it's a bit like Kotlin. I heard though that C#'s move still has some std lib bits that are nullable.

The idea is not to never have nulls. It is pointless (ha) - the way to understand T? vs T in C# is like an optional.

The entirety of standard library is annotated since long time ago. All new and not so new projects are also null-aware. Pretty much either completely legacy libraries or libraries that explicitly removed Nullable: enable that is set by default for all new project templates do not have those.

As I mentioned previously - it isn't perfect, but the level of "good enough" of NRTs in .NET is such that the nullability is a solved problem.