UIScrollView和自动布局不起作用

As mentioned in the video, the main aim is to automatically adjust the UIScrollView contentSize to the size of the screen or to the size of the content in such way that on smaller devices scrolling is active and on larger devices scrolling is not active because it is not needed.

For this to achieve you've to set proper constraints to the content view so that the height is automatically calculated. From your video its visible that you're not setting the proper constraint to the last UITextFeild (this is important).

您必须在最后一个约束中赋予顶部和底部 约束，UITextField以便自动计算contentView的高度。

我已经完成了一个示例项目，您可以在此处查看：
示例项目