Primary category "auto"

Hi,
if I set the Primary category “auto select a category” based on what criteria is it set?
I thought that the category with the most posts is automatically set, but it isn’t.
I would like to set the category with more posts.
I have created many categories as “featured” in order to better manage the order of the posts on the homepage (I use many post blocks with multi categories).
Thank you

Thanks for your answer.

I think it would be better to set the automatic primary category differently

Is there a way to exclude some categories from the automatic primary category selection?
But the same categories must be able to be selected manually via the metabox in the post edit.

I use this method to set a different layout for “big news”, but I would like to avoid having this different layout set automatically based on the alphabetical order of the categories. And I want to avoid setting it manually on every post.
Hope you explained correctly.

Thanks.