Ask HN: How to implement caching for dynamic user data in sites like HN, Reddit?

Hi guys. Need some thoughts on how can implement caching for sites like HN, Reddit etc which have a lot of dynamic user metadata (eg. liked comments)?

Earlier on, when I only had the user name ("Hi User Alpha") on the navbar, my plan was to send the entire HTML which looks the same to both logged in and non logged in users. Then to use Javascript to add the user info which I could get from the browser cookies.

However, Now I am looking to add more features like comments with likes and tags (with voting for tags), and I am confused as to how can I cache any of it. How do sites like HN, Reddit etc cache such dynamic user data?