I love using macOS as my personal OS.
And I really like NixOS and it's nix package manager.
Introduction to Operating Systems
OS: Three Easy Pieces
mirageOS - A programming framework for building type-safe, modular systems.
Raspberry Pi OS - Learning operating system development using Linux kernel and Raspberry Pi.
Create OS from scratch
How to Make a Computer Operating System
Plan 9 from User Space - Port of many Plan 9 libraries and programs to Unix.
Jehanne - Simple operating system.