The post linked discusses the basics of set theory. Part 2, which goes into how set theory is used for constructing numbers like the square root of 2, is here: <a href="https://vinaybhaip.com/blog/2020/11/22/sets-to-numbers" rel="nofollow">https://vinaybhaip.com/blog/2020/11/22/sets-to-numbers</a>.