Understanding WordPress(framework?) Web Development

I have come across a wonderful portfolio website https://dogstudio.co/ while I was looking for examples of design. Upon exploring the source-code, it was apparent that this was a wordpress website after all.

What I don’t understand is how and why does it not have the typical wordpress directory hierarchy. For example, this is the URL to their CSS file, https://dogstudio.co/app/themes/portfolio-2018/static/css/main.css.

A typical wordpress website would have something like this, https://www.example.com/wp-content/themes/theme_name/style.css (the wp-content and wp- prefixed directories) as a general rule of thumb for making wordpress themes.

So was it made using a framework? I really need to understand this.