Ask HN: Best CS refresher material online?

Recently I've been looking for a good "refresher" for the CS theory I learnt at university - in particular the theoretical, slower-changing stuff that we had to learn but rarely apply in a web programming career.

What sparked it was actually an episode of University Challenge where there was a mention of Byzantine Generals - I haven't heard that mentioned in about 18 years!

Though any professional development I get from this is a bonus, my main motivation is just intellectual curiosity & the desire to re-explore these concepts that I slowly forgot through disuse over the years. As a result, material that is captivating and fun is a strong bonus.

Any books, podcasts, youtube channels, online courses HN would recommend?