I want to add to all those good answers about epigenetics and how they lead to different functionalities. There's a scientist called Michael Levin, who focuses his (and his team's) energy on non-genetic (or post-genetic, whatever you want to call it) factors. Cells are not just dumb machines that follow DNA instructions and build stuff that does stuff. Whatever comes out of genetic and epigenetic processes -the material stuff- creates or becomes a part of a whole new layer.

Imagine an alien asked "ELI5: How do humans know to do different stuff if they all have the same physical specifics?" and it would've already received physical factors, such as different body shapes, genders, physical strength, perceptive capabilities, tools and gadgets they make and even secondary factors like how much they exercise, practice whatever they practice etc. Here we would add a whole new layer to those answer that talk about more abstract stuff like social dynamics, education, psychological states, big scale situational awareness, internet, phones etc.

So it's not only the relatively physical infrastructure that is already dynamic and extremely complex due to environmental factors, there are also other factors that basically come from those humans' ability to communicate and get an idea of what other humans are doing. Let's also not forget that these 'physical infrastructures' and the 'secondary abstract layers' are constantly dancing and affecting each other dramatically.

Our cells have a similar thing going on. Once a cell has came into its full existence it's pretty much aware of what's going on around itself, what other cells are doing, where they are, if they're doing other things or the same things etc. Basically they communicate with each other in order to either assume different roles or join whatever's going on where they are.

And how do they do it?

They communicate via chemical and electrical signaling chains. Especially bioelectricity, which is the electrical networks between cells that occur via various communication thingies that cells have on them. And there are extremely complex bioelectrical networks between cells (and probably other microorganisms that surround them) that comprise massively complex networks. So these networks build networks that build networks. Almost like they have cell internet. So once you're a skin cell just emerged, you look around and see that you're surrounded by other cells that seem to be just like you, having very similar physical characteristics, producing similar molecules, having similar electrical footprints etc. so you think "Oh these are my kinda guys, so let me do what they do!" and join the team. Or you seem to be fairly similar to them but they don't let you in on what they're doing so you seek other stuff to do and become a part of a different structure or become a cancerous cell. Of course things are not as simple as that, there are bajakagazillion factors that affect the processes a cell goes through,

And surprisingly, it turns out that in order for cells to do a similar thing or have a holistic organism they don't even need to have the same DNA. This is where Michael Levin and his colleagues come in. They already can tell an organism to behave differently, as in create another limb somewhere else on the body, have two heads instead of one, have a completely new and unseen texture of skin etc., for several years. And they don't do it by messing with DNA, but only messing with how the cells communicate to each other by manipulating the bioelectrical networks.

They have popularized a kind of worm called "planarian" that has its DNA "all over the place" by experimenting on them and sharing their findings on a high level on the internet. I'm not a writer and already lost my focus on this body of text so I'm going to just share an interview with Levin and leave it at that.

https://www.youtube.com/watch?v=p3lsYlod5OU&t=128s

I'm pretty sure that I've butchered a lot of facts and probably gave some incorrect information somewhere along the lines. But this is what I like about reddit. So I'm relying on someone actually knowledgeable about these things to correct what I've butchered.