Rust - A much beloved systems programming langauge focusing on safety (and wonderful error messages).
The Book - An introduction to Rust
Rustlings - Rust through small exercises
The Rustonomicon - Peeking underneath the hood.
Rust with too many linked lists - Basic and advanced Rust illustrated through linked lists.
Zig - A simple, ergonomic systems programming language which integrates with C, Objective-C and C++. Zig also has a built-in package manager and a build system.
Ziglings - Zig through small exercises (like Rustlings).
Git from the bottom up - A brief look at git internals
Building Git - James Coglan highly regarded book which develops a git clone.
Well not literally git clone git@github.com:git/git.git.
Pijul - Pijul claims to be based on a sound theory of patches
Pijul is apparently inspired by a result from category theory.