Is an Office a custom post type [closed]

How you store your data has nothing to do with whether it is add/edited/removed from a plugin or your (child) theme’s functions.php file. So I’d say that your question is whether to add a custom post type or use a custom table for you ‘office’ entity.

You being new to WordPress, I’d advise you to go with custom post types. And use some plugin to create and manage it (like Pods or Custom Post Type UI). You can start off testing and making changes visually, which I think will be very convenient in your case. Then you can opt to implementing it with your own code.

Those plugins also allow you to add custom taxonomies for your added custom post types, so you should be good to go.