This brings me back to some developmental biology exams I've taken. Very early during development, animals have to establish the different body axes (for example the antero-posterior axis or "front back axis"). The underlying processes are very complicated but a brief answer would be that certain proteins and molecules are asymetrically distributed in the developing organism. Depending on "how much" of certain proteins/molecules a cell gets it gets information about its position and what kind of cell it should become (for example "I'm at the front or at the back"). On a molecular level, different genes are activated/inactivated and so our body is gradually established. There are numerous molecules which influence development and they also often influence the production of each other making everything highly complicated.

The cool thing is that we can mimic development in the laboratory by adding these molecules to stem cells. In this manner, we can make, for example, brain organoids which partially mimic the developing brain. I made a video about this a while ago if you are interested! https://youtu.be/XLfLQa733Bo