How exactly are you supposed to learn how to code games

I understand CS principles and can code basics scripts but when I look at stuff like pic related, which is a YouTuber coding a terrain generator in Unity, I get overwhelmed and wonder how are you supposed to source this knowledge from scratch

Like if I wanted to implement my own version of a terrain generator or make alterations to this one, not only would I not know where to begin, I wouldn't know where to start learning to get on the path of knowing where to begin

Are there any courses that would bring me up to speed on how to think when programming in C# for Unity