I'm new to using WordPress and so, apologies in advance if this seems like a very basic question.
I want to have multiple authors for single post. I had seen Co-Author. But I don't have to use plugin at all.
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No need to apologize, and not a basic question actually. Depending on what you actually need to achieve once you have your multiple authors assigned, there are several ways to do it:
If you just need to display "authorship" for any given post, you could create a custom non-hierarchical taxonomy (or even use tags if you are not using them in your theme already), and use those to actually "tag" an entry with multiple authors.
That would not give you by default the full WP Author backend capability (meta data, author info, etc), but you could certainly and easily filter by author within your theme's loop to display all posts by that tag/author.
Similar to above in that you have to manually assign each new author to a post, but much less powerful and cumbersome, as you would then need to create functions to retrieve and filter custom fields within your theme
Within you functions.php you can create new functionality for your theme. In this file you can create code to display UI elements under WP Admin pages, and also functionalities you can then call from your theme. If you want to learn how to do it, I'd even recommend using the co-authors plugin and trying to port it into your theme functions.... but then, you would be using a plugin.
A plugin is nothing more than functionality you could have in your functions.php file, that was wrapped as a plugin as an easy way people you to detach it from your particular theme and reuse it in different ones.