Terence Tao has uploaded his slides on the Collatz conjecture (targeted at high school students): https://terrytao.files.wordpress.com/2020/02/collatz.pdf
A very enjoyable read indeed.
The best “encyclopedic” reference on the Collatz conjecture is the one listed below, published by the American Math Society. Note that the Collatz conjecture remains unsolved as of today.
The Ultimate Challenge: The 3x+1 Problem
Singapore Maths Tuition 