Nice summary. I've been using Ruby both professionally and not for going on 20 years and I just today learned about singleton_class when attempting to build something like Rails' view helpers from first principles.

Thanks. I've been trying to put these bits in a succinct format:

https://lloeki.github.io/illustrated-ruby/

Each chapter kind of builds up on the previous one. It's still WIP and far from complete but what's there has helped me onboard a few people to Ruby already.

You might be interested in the Classes and Ancestry chapters.