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
78 / 295 pages

The Rust Programming Language
200 / 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
 
