If you're interested in a step-by-step guide on how to build your own git from scratch, this book is excellent and incredibly educational: https://shop.jcoglan.com/building-git/
For the curious, this guide uses Ruby as the building language.
I've also seen a number of people following along in this book in other languages, including Rust. It really is a well written guide, even if you're not fluent in Ruby.
Trust me its way easier than you think and there are tons of free resource available you dont need a $36 book to build git.
You probably misunderstood: they're not talking about building from source (which is easy), but rather about recreating Git from scratch in another programming language (which is hard).
I didn't watch the video but I've gone through that guide you linked, it's excellent but I don't think it's a great comparison. It only implements a handful of commands and doesn't go into the same amount detail. The Building Git book goes way more into the interesting bits.
My whole point was it is way easier than one thinks and there are good free resources available. Once you do the basic of git filesystem , object storage and sha basics implementing the rest of the API is trivial in comparison.
It's true that you can find most content somewhere freely available in various disparate corners of the web and everything is trivial if you have the info... but that's a strange reason to automatically reject a book you haven't read.