❔ does an interface go inside a generic parent class?

using Godot;
using System;

public class StateTemplate : Node
{
    public enum State {
        Null,
        Idle,
        Run,
        Fall,
        Jump,
        Dodge
    }
    [Export]
    private String animation_name;
    public Player Player;

    public void enter()
    {
        Player.AnimSprite.Play(animation_name);
    }
    public void exit()
    {}
    public int processInput(InputEvent @event)
    {
        return (int)State.Null;
    }
    public int doProcess(float delta)
    {
        return (int)State.Null;
    }
    public int processPhysics(float delta)
    {
        return (int)State.Null;
    }
}

here is the parent class for my state machine, all states will have a processInput() and processPhysics, do I create an IState interface inside or outside of parent class? I ask because I am translating this from another language and I think this is a pseudo interface