Ask HN: Best resources for refreshing CS fundamentals?

I've been working full-time as a software engineer, mostly on frontend/backend web development, for 7 years now. I've mainly been working with Python and Javascript.

It feels like a long time since I finished my Computer Science degree and I'd really like to go a bit deeper with my low-level CS fundamentals.

I've started doing a bit of a C refresher, which feels like a good start.

I'd particularly like to improve my knowledge of the full networking stack, operating systems and hardware. I'm open to other suggestions of areas to work on, too.