Tesla invented battery management technology to scale up bundles of small lithiun ion cells. This requires significant engineering to get right: cells charge and discharge at different rates, heat must be managed and failed cells taken out of the stack. Nobody was doing this commercially when they built the first roadster. IIRC they shared this tech with Toyota pretty early on.

This is all basic electrical engineering, and now reproduced by other carmakers, but Tesla did it first.

For example, when I was at National Semiconductor working on Solar tech (2010ish), another team was building systems for lithium ion battery management for cars. They did this to sell the tech to car makers.