If it's truly a non-Flarum page, you'd have to pull the session cookie, boot the PHP session, extract the user ID from there and then do your stuff. I don't recommend doing that as it's complex and could break with future Flarum versions.
If you are going to develop such a page from scratch, I recommend creating a new Flarum page in the single page app. That way you'll have access to everything the Flarum session offers and can easily make requests to the Flarum API. As Flarum is a single page app, such a page is developed in javascript.
Your description of a page sounds very much like the index page of Flarum. What would be different and what would the purpose be ? You might be able to simply theme the homepage instead of creating a whole new page that does the same.
