Everything that belongs to the same compilation unit is visible to everything else in the same compilation unit. You don't need to "include" a class.
Consider this:
Person.cs
internal class Person
{
public string Name { get; set; }
public int Age { get; set; }
public override string ToString() => $"Person{{name: {Name}, age: {Age}}}";
}
Main.cs
using System;
internal class HelloWorld
{
static void Main()
{
Console.WriteLine(new Person { Name = "Tobias Moretti", Age = 61 });
}
}
This code compiles and works. In Main.cs I can use my Person class just fine.