system
Given the following post, respond as if you're commenting on Reddit:
human
How does a stem cell know what body part to become naturally?

What type of communication happens inside an embryo? What prevents, lets say, multiple livers from forming? Is there some sort of identification process that happens so a cell knows "okay those guys are becoming the liver, so I'll start forming the lungs" ?
gpt
The process you're referring to is called "Differentiation", which in biology refers to stem cells becoming other cell types.

Chemically: The chemical properties of the surrounding the cell and available nutrients play a factor.  These are things like pH, oxygen levels, CO2 levels etc.

Physically: Certain cell types are receptive to different mechanical stimuli and surface properties.  If you're trying to grow stem cells on a certain material you can alter it at the nanoscale to promote differentiation into a certain cell type.  Another example is repetitive stretching and compression can promote osteoblast (Bone forming) cell differentiation.

Biologically: There are tons of biological signals that can promote differentiation in one form or another.  Cytokines, proteins, antibodies, hormones etc.  There is a common technique called 'Coculturing' where a scientist will grow stem cells and another cell type in the same media.  The proximity to the second cell type can determine what type of cell the stem cells ultimately become.

Preexisting factors: Not all 'Stem Cells' are the same!  There are several classifcations.  "Totipotent stem cells" can differentiate to become just about any cell type or expand to make more stem cells.   "Pluripotent stem cells" can become almost every cell in the body.  "Multipotent stem cells" can become several different types of cells, but not all and are more limited than Pluripotent.

This is still a rapidly growing and changing field, and there are certainly blurred lines between those classifications as we learn more about cellular differentiation.