BaseState.cs

public abstract class BaseState 
{
    public virtual BaseState Update()
    {
        return null;
    }
}

public class IdleState : BaseState
{
    public override BaseState Update()
    {
        if(移動の入力があれば)
        {
            return new MoveState();
        }
        return null;
    }
}

public class MoveState : BaseState
{
    public override BaseState Update()
    {
        if(移動していなければ)
        {
            return new IdleState();
        }
        Move();
        return null;
    }
     
    void Move()
    {
        //移動の処理
    }
}

Main.cs

using UnityEngine;

public class Main:MonoBehaviour
{
    BaseState state;
    void Start()
    {
        state = new IdleState();
    }

    void Update()
    {
        BaseState currentState = state.Update();
        if (currentState != null)
        {
            state = currentState;
        }
    }
}