Getting user data from WP cookie and then using it

I know WordPress stores user info in a cookie somewhere. I’ve read WordPress Cookies, wp get cookie login, and get profile, but since I don’t know how any of that works it doesn’t make any sense to me.

I need to get either the username or the user ID from the already logged in website visitor, and then be able to use that info on a custom coded, totally non-WordPress page. I’ll worry about how to use that info later (possibly on a php forum instead of here) but first I need to know how to access the visitor’s username from a totally non-WordPress page. How do I do that first step?

(I’ve also seen that I can do something called sessions. Not sure how that works but in the end I just want to pass along info about who is viewing a non-wp page. Are sessions a better way to go?)