and first of all sorry for the length of the question and my english.

I need to modify only the category posts page in my wordpress theme (OceanWP), not single post page (it's already perfect as it is).

I partially achieved the result I wanted but I reached that result via CSS and all margins are subject to changes based on title article length and image size when I post a new article. I basically changed the category posts page via CSS, as you can see on the page, but the results may vary and not be formatted correctly.

So, which file I need to modify on my WP child-theme to modify only the blog-entries page? I can't understand it. On my child-theme only have functions.php and obviously style.css (again in OceanWP folder theme have also page.php footer.php comments.php and others).

Here you can see my layout obtained with CSS for one of blog's categories (each category must have the same layout, logical):

/

I'd like to obtain the same result via template (php file?) to have a page formatted correctly.

On my layout I need:

1. the first post (full page width like now): one row and one column with featured image on background (now it's a thumbanil, it's grainy and blurry as you can see, if you open the post it's perfect instead beacuse it's original size).
2. the second post: one row with one column width 970px (post title and excerpts on the left of the thumbnail)
3. third post: one row with one column width 970px (post title and excerpts on the right of the thumbnail)
4. 4th 5th 6th posts: one row with 3 columns (970px total width of the 3 columns)
5. 7th 8th 9th posts and so on....

I hope I explained myself.

Then here begins the fun, the php code to use to get what I would like to display the posts page, but this is another matter, first of all I wanted to understand "where to start from" and on which file make changes. And if anyone would like to give me some code suggestions thank you very much in advance advance.

Ric