Short answer is no one knows, but there's tantalising evidence that it has to do with macro extracellular electric gradient in tissue: https://www.youtube.com/watch?v=RjD1aLm4Thg

Every cell contains the entire genome which, informed by chemical messages from cells nearby, has a subset of genes activated which encode the information needed to know what to do.

Even the simplest cells on Earth are extremely advanced and intelligent, the latest in 4 billion years of evolution

It's fascinating how the same type of cell is used throughout the body to make different body parts. It's like nature came up with an abstract <Cell /> component that it reuses to compose a body.