Where is the line between idea theft / use of copyright material and using it as "inspiration"?

It is, of course, a line that is very complex and the answer is very nuance-based.

Let's take webdesign for example; this site ( https://thomasdigital.com/the-legalities-of-copying-a-website-design/ ) says the following;

"You cannot duplicate copyrighted elements such as images, text, or source code.", as well as "trademakr and logo".

but there some exceptions such as accidentially copying a design.
And here comes the interesting part...
He says that "There is nothing wrong with looking for inspiration on other websites. You can point out elements to your web designer and [notice here] even incorporate them into your design. You just don’t want to copy the elements themselves or the code … Just make sure it doesn’t scroll the same images, and your web designer uses their own code to create the element and doesn’t copy the code from the website you found."

but before that he says that, "Web developers write custom code for one client. Therefore, they own the rights to that design. It does not matter if another site [notice here] copies part or all of your site. If any of the elements of your custom website are reproduced on another site, then you have a case for copyright infringement."
From copyright infringement regardless of copying parts or all of your website to "even incorporating them into your design"

Now, lets say that I liked look and structure of the top part of the notetaking desktop app OneNote. Could i then use the exact same design structure, but not directly copying it, and also switching the color from purple to black, AND using my own software code?

Sorry for the long text, i appreciate if you could clarify this to me.