So, when each page has content on it's own language we use HREFLANG attributes and it's all fine.
But now, if I build a page in English but if user picks to display it in another language, I won't open a new URL of the alternative page in that language, but rather dynamically re-render the page with contents in requested language. URL won't change, contents will. And later, if that user returns to the site, i'll automatically display site contents in his language...
How do I SEO such pages?
I know that I'll need to use vary accept-language, but what else?
Also, if I had to redirect from old Hreglang versions of URLs to this one-of-all URL, would I just 301 redirect those pages to it and auomatically display preset language content according to origins of redirect?