I've been diving into this operating systems course https://classroom.udacity.com/courses/ud923/ in my free time,

It's about 9 years old I think, but it's been very enjoyable

Earlier this year I embarked on a project to help me learn how CPUs work, I ended up implementing simulator for a simple CPU from the "But how do it know?" book - I have a blog post about it if anyone is interested.

Since then I've been dabbling with learning about RISCV, but it's been slow progress.