Different Theme for Different User/Categories Display

1. You mean you want to select for them or they should select it?

2. I am not sure I understand this one. You want a category to be visible to only 1 user?

For new users it is always useful to take a look at the zillions of good articles on the Codex the official documentation site.
I’d recommend: https://codex.www.ads-software.com/Getting_Started_with_WordPress
https://codex.www.ads-software.com/Working_with_WordPress
and later you’ll find the rest ??

I mean, that there would be a URL that only displays the postings in one category from ONLY one specific user.

In regards to theme…it doesn’t matter who selects it. I just want the layout/formatting for one user to be different than that of another user.

Thank you.

Oh, I am slow today, or just need some sleep ??
By “user” you mean an author, right? You can list the posts of one single author (check out the Template Tags section and Pages in the Codex), but I am not sure it can be done to show only his/her posts from a certain category.
Still confused about the theme question. Do you mean to display the posts of an author with a different theme and the posts of another author with another theme?

Yes you are exactly correct – but this is assuming you are viewing a page with ONLY the postings from THAT user. Not a page that has multiple users.

You seem to be pretty experienced in th is…is there a way I could contact you for more rapid communication..I have a greater scheme in mind that, if you kindly permit me, I’d like to share with you.

Thanks.