// We are in MonoBehaviour script
private void Update()
{
    // We can cache this Vector on Awake, but still, every script has new vector
    // So it's 12 bytes (3 floats each 4 bytes) PER SCRIPT from vector
    transform.position += new Vector3(0, 1, 0);
}
// We are in ComponentSystem
struct Group
{
    public readonly int Length
    public ComponentDataArray<Position> Positions;
}
[Inject] Group data;

protected override void OnUpdate()
{
    for (int i = 0; i < data.Length; i++)
    {
        // When Unity uses ref returns from C#7 it'll make the extra assignment redundant. 
        var position = data.Positions[i];
        // We can cache float3 so we'll have only one variable
        // If so, in contrast to MonoBehaviour - it's just 12 bytes OVERALL - we have it cached in system
        position += new float3(0, 1, 0);
        data.Positions[i] = position;
    }
}

• Position - { float2 Pos } - must have for ducks and our crosshair to place them where we want
• MouseInput - { bool Fire, int2 Pos } - we will check where is mouse position and when fire button was pressed
• Health - { float Value } - some ducks you can oneshot, others can be stronger
• MoveSpeed - { float Value } - some ducks can be faster than the others
• DuckTag - { } - empty struct, just for proper filtering
• PlayerTag - { } - empty struct, just for proper filtering

• SpriteRenderer { Material mat, Image img } - we don't want to shoot to invisible ducks, do we?

• InputSystem (operates on group of { MouseInput } ) (Behaviour => MouseInput = Input.Mouse, Fire = Input.ButtonDown)
• CrosshairSystem (operates on group of { MouseInput, Position, PlayerTag } ) (Behaviour => Position = MouseInput)
• MoveSystem (operates on group of { Position, MoveSpeed } ) (Behaviour => Position += MoveSpeed)
• ShootSystem (operates on group1 of { Position, Health, DuckTag } + group2 of { Position, MouseInput, PlayerTag } ) (Behaviour => if(group2.MouseInput.Fire && group2.Position == group1.Position) group1.Health -= 1)
• DuckDeathSystem (operates on group of { Health } ) (Behaviour => if(health <= 0) Destroy(thisEntity))
• GameConditionSystem (operates on group of { DuckTag } ) (Behaviour => [Always update] (attribute that prevent group from being disabled it there is no matching entity); if(group.Length <= 0) GameOver())

• (Player(Crosshair)) - has components { PlayerTag, Position, MouseInput }
• (Duck0) - has components { DuckTag, Position, Health, SpriteRenderer, MoveSpeed }
• (Duck1) - has components { DuckTag, Position, Health, SpriteRenderer, MoveSpeed }
• (Duck2) - has components { DuckTag, Position, Health, SpriteRenderer, MoveSpeed }
• (…)