There's a lot more D3 can do, but those are the 3 key insights you need to understand any example you find in the wild.
- Code is split into data and DOM manipulation
- Scales are great and used a lot
- You're always in control of rendering
Then you can start using D3's more advanced features like maps and axes and shape generators and geo computations and data loading and transitions and user interactions.
There's a lot. We're not going to cover it all but you can find those advanced features in the docs and the skills you learn here will help you get started.