Why Singleton class needs to be sealed?
https://dotnettutorials.net/lesson/singleton-class-sealed/
In short, internal derived class will be able to create instances.

.NET DI framework list.
https://www.claudiobernasconi.ch/2019/01/24/the-ultimate-list-of-net-dependency-injection-frameworks/

Singleton vs Dependency Injection.
https://enterprisecraftsmanship.com/posts/singleton-vs-dependency-injection/
In short, Singleton is better for ambient dependencies, i.e. dependencies used everywhere