Q: What is global namespace? The global namespace is the namespace that contains namespaces and types that aren't declared inside a named namespace

Q: How to use? ::global

Ref: https://learn.microsoft.com/vi-vn/dotnet/csharp/language-reference/operators/namespace-alias-qualifier

In C# 9.0 and later versions, when you use top-level statements in a Program.cs file, there is no default namespace defined.

IF "there is no default namespace defined" = "without any namespace declaration" -> the code in Program.cs belongs to the global namespace since there is no default or explicit namespace specified in the file