Create instance using an interface

I use this way

interface IObject{
    first: number;
    second: string;
}

then

var myObject = {} as IObject
var myListObject = [] as Array<IObject>

You can do it that way. You can also just create an object that implements the interface like:

interface foo {
    one: number;
    two: string;
}

const bar: foo = { one: 5, two: "hello" };

If you want to use a class, you can put it where you want. If it's tightly coupled with the component, you can put it there. Generally though, I want classes to be loosely coupled, so I put them in their own file.