What happens *inside* a neural network?

From a channel Lumen includes in full: vcubingx (Vivek Verma) makes animated maths and machine-learning explainers in the 3Blue1Brown style, covering topics such as Fourier series, complex analysis and neural networks.

A 14-minute video from vcubingx's catalogue; the title names its topic.

Video by vcubingx, played through YouTube's embedded player.