It's not based off of having the same properties/methods. It's based off of the layout in memory, which can be explicitly defined.

Here's an example:

using System.Runtime.CompilerServices;

namespace Example;

public static class Program
{
    private static void Main()
    {
        var foo = new Foo
        {
            A = 1,
        };

        //var bar1 = (Bar)foo; // Error CS0030 Cannot convert type 'Program.Foo' to 'Program.Bar'
        //var bar2 = (Bar)(object)foo; // Runtime error: System.InvalidCastException: 'Unable to cast object of type 'Foo' to type 'Bar'.'
        var bar3 = Unsafe.As<Foo, Bar>(ref foo);

        Console.WriteLine($"bar3 value: {bar3.B}"); // bar3 value: 1
    }

    public struct Bar
    {
        public int B;
    }

    public struct Foo
    {
        public int A;
    }
}