Microsoft.Extensions.DependencyInjection is combined with a core DI abstraction under Microsoft.Extensions.DependencyInjection.Abstractions that allows for building different kinds of dependency injection containers to retrieve services from that have been registered with different lifetimes.
Documentation can be found at https://learn.microsoft.com/dotnet/core/extensions/dependency-injection.
The APIs and functionality are mature, but do get extended occasionally.
Microsoft.Extensions.DependencyInjection is not included in the shared framework. The package is deployed as out-of-band (OOB) and needs to be installed into projects directly.