Not a repo but I remember a book titled something like "architecture in 500 lines or less" that had really pretty explanations and diagrams

Was it this one: https://github.com/aosabook/500lines ?

There's a small asterisk to your question: there's no way to verify that the diagrams represent the underlying system without access to the code (and sometimes the deployment). Anyone can make pretty pictures without spaghetti lines, but it takes some real, no kidding, discipline to write code (and then deploy that code) in a non-spaghetti way

It's like that phrase "no plan survives first contact with the enemy" but applied to product owners and then subsequently applied to computers

100% correct. The only ones which had both a repo and a diagram that I've consistently found were AWS sample architectures. Their diagrams are usually pretty lacking, though: https://www.ilograph.com/blog/posts/better-aws-architecure-d...

This is, of course, extremely subjective, but I find myself using this slide deck from Google Cloud (https://docs.google.com/presentation/d/1fD1AwQo4E9Un6012zyPE...) as inspiration and/or to see how others design their diagrams.