One of the beautiful things about brewing is the pleasure of making something manually, in which you put some thought, and that you then can enjoy.
As part of the thought you put into it an understanding of the process and the principles is required.
That understanding needs to be communicated, in order to verify it and also to enrich it against other peoples ideas.
In the case of brewing the process is simple. Yeast, which are unicellular organisms, need to eat to survive, just like us. We eat all sort of different things since we are complex animals, but for a yeast, some sugar, and minimal amounts of protein are enough. The rest they manufacture themselves.
Yeast being very simple they do not have a complex mechanism for eating. They just float in water, in which their nutrients are floating, and the nutrients enter the cell membrane according to a difference in concentrations.
If we were to imagine it in humans, we would be floating in a soup. We would not eat as such: food, drink and air would just enter our bodies and be consumed.
We all know that what we eat produces waste. That is also true for yeast.
The same way the the nutrients are internalised across the membrane, the waste material which are the toxic by-products of transforming the nutrients, are also eliminated across the membrane. As the yeast eats, it grows, just like us. As it grows it creates little yeast cells on the edge of the membrane which when sufficiently big break off and become other yeast cells which then again float in the same nutrient liquid.
If at this stage you have been following, you would have realised that the yeast cells float also in their own waste material. So to bring back our analogy we would be in a soup from which we eat, and at the same time defecate in it. That may seem indeed gross to us, but a yeast being a single cell has no neurons, hence can not develop advanced concepts such as gross.
Most yeasts, if the medium is not refreshed would die due to the waste that they generate, same as we would. However some yeast are resisting to their waste. Some of them are alcohol resistant.
Oh yes, I forgot to mention, yeast eat sugar, but defecate alcohol. Is that not a cool thing?
Yeasts are happy eating and reproducing, eating away as much sugar as they can, and on top of that
they poop alcohol. Which we actually appreciate because of the effects it has on our brains, and our bodies. Yeast as part of their growth also fart, just like we do. We expel methane which is the by-product of some bacteria that lie in our gut. Yeasts expel CO2 which we also appreciate because it tickles our tongues when the bubbles break away from the liquid.
So in effect beer are the left over nutrients that the yeast could not eat, together with what they farted, CO2, and what they pooped. The best thing of it all is that we love what those little yeasts do.
At the end of the day all we aim to do is, make the yeasts happy.
That is provide them with a lot of food so that they eat a lot and poop a lot.
This is obviously a slight simplification, not all the sugars break down equally, and that gives the aromas to the final product. But you do get a pretty good idea of what you drink.