What is the best way to get started with D3?

I would start with an fairly recent tutorial written by Bostock himself: https://observablehq.com/@d3/learn-d3

A point I personally needed to grasp back when I first worked with D3 was that the data drives the visuals (hence data-driven documents). It was not an animation library. It's also not a charting library, but some charting libraries are built on top of D3.

Lastly, the D3 wiki also contains links to other tutorials, but they're most likely running on v3:

https://github.com/d3/d3/wiki/Tutorials

I have recently taken up a project to build out a dashboard for realtime data analytics. I did some exploratory research of the available data analytics frameworks mainly in Python. I found Plotly to be very popular. How much extra work would it be to just use D3 on the frontend with a standard python web server on the backend versus using just the Plotly library?

I liked this book:

https://www.newline.co/fullstack-d3