In addition to the Ruby under a Microscope (linked elsewhere), a lot of talks from Aaron Patterson (Tenderlove) are enlightening in this regard. I've learned a lot from them, and they also provided the inspiration to keep digging. Some highlights:
Memory:
- https://www.youtube.com/watch?v=ZfgxvNUfQdU&t=3s&ab_channel=...
- https://www.youtube.com/watch?v=1F3gXYhQsAY&ab_channel=Confr...
JIT and the Ruby VM:
- https://www.youtube.com/watch?v=FCjwSOlHqbY&ab_channel=hexde...
- https://www.youtube.com/watch?v=5QgQicKHmeU&ab_channel=RubyC...
And loads of others.
Probably this book https://www.amazon.com/Ruby-Under-Microscope-Illustrated-Int... will be sufficient for you.
I had a look around last time I dived into Ruby internals, and there's really not a lot around. But the Ruby source is well structured and well commented, I'd recommend just piling in to it, when you get stuck search on the text of the comments or ask on SO etc.