a singleton in aspnet core allows multiple instances of the same programming language

I believe the problem is how you’ve defined the Id property on your Person class:

public class Person : IPerson
{
    public string id => Guid.NewGuid().ToString();
}

This means: every time the code asks for the id – it gets a newly created Guid – so of course you keep getting new/different Guid’s every time you look at your person!

You need to get the id once – in the Person constructor – and then return that:

public class Person : IPerson
{
    private string _id;

    public void Person()
    {
        _id = Guid.NewGuid().ToString();
    }

    public string id => _id;
}

I’m pretty sure once you’ve done this, the singleton will work – you keep getting back the same instance of Person every time you look at it.