I have found that books are the best source of knowledge especially for low level concepts. I have been spending a lot of time finding and reading books in the fields I am interested in: operating systems, programming language theory. Some of them are listed below.
Currently Reading

Learn Concurrent Programming with Go
66 / 295 pages

The Rust Programming Language
104 / 527 pages
To Read
- Writing A Compiler in Go by Thorsten Ball
- Crafting Interpreters by Robert Nystrom
- Operating Systems: Three Easy Pieces by Remzi and Andrea
- Designing Data Intensive Application by Martin Kleppmann