We used to have dedicated 50 Gbps fiber connections straight between the web and memcached nodes, but now the caching is done locally on the web servers with a lot more of them. I would say there's probably something like 100+ servers, 128/256GB RAM each.
All the pages are server-side rendered. Every DB query, every DAO object instantiated, every partial/full page HTML render, even some functions at times, gets put into cache. We weren't really afraid of high traffic, that scaled almost on its own... instead we were afraid of infrastructure maintenance or bad manipulations, because too much of the cache could disappear at once.
The most incredible part to me was the storage and delivery of the video content. Try to picture 7000 petabytes [3] :)
[1] https://www.pornhub.com/humans.txt [2] https://github.com/nitrix [3] https://www.pornhub.com/insights/2019-year-in-review