Insects work mostly automatically, their actions get triggered by specific impulses.

There was an experiment where they observed a beetle doing his mating... Things. I don't remember what exactly he did. Anyway, by disrupting the task and putting the bug in the same place as before, it would start the next action again and again, not realizing that it's attempts were futile. The bug could not learn and adapt to the situation. It was forced to play through its program linearly.

This cant directly be answered with fact, but this is my understanding of it:

There are two different kinds of "things" you know to do when the time comes.

The first kind is related to immediate needs: you feel hungry, your body knows it needs food. You feel the need to use the toilet? Your body knows it can expel waste to relieve the feeling. Note that these things are basically the same, as your body feels something that isnt the norm, it tries to figure out what is wrong and how to deal with it. Its really as simple as this: some individuals of a species expelled waste when they felt "a feeling", and therefore had a lower chance of dying from the eventual buildup of waste. Eventually, as evolution favours this connection (waste is full -> go poop) it is nearly ingrained in your species as a whole.

Second, there are "milestones" and "cycles" As one grows older and matures, the body changes at different times depending on the species. In the end, evolution (and by assocation, life itself) exists with the sole purpose of reproduction. Every single species of living organism today has undergone thousands to millions of years of optimization in terms of when and how to reproduce. For humans, it happens that around 13/15 years is enough for us to grow big enough and self-sufficient to begin proritizing reproduction. By this age, we are likely to survive on our own to the degree that we have enough "resources" left after taking care of our own needs to also successfully raise another individual. (Note that "resources" refers to all of everything you do: access to time, energy, food, safety, whatever the creature has or needs) Because of this, evolution has figured out "okay so we have 85% success rate of reaching 12 years of age, and rarely die from natural reasons at that point. Lets have the body start prioritizing reproduction" and kicks in puberty by reallocating some resources to maturing the reproductive system. Im massively simplifying of course, but its all about cause and effect in a long chain resulting in what we have today. Hormones are chemicals, and specific chemicals bond with other specific chemicals in specific ways. For most organisms the logic has come to something like this: Age x reached -> start reproductive development/puberty -> prioritize reproduction

So in a way: some things happen at specific times because evolution has at one point found it optimal. What you "know" to do is simply your body making logical conclusions from the information you have, encouraged by all of your ancestors doing the same for thousands of years. At some point, some individuals made a connection "i feel x, i should do y" and it was passed onto offspring. If the logical conclusion is sound, and it helps boost reproductive success, then the logic sticks around in coming generations.

TL; DR: Everything in parentheses is simplified ELI5 as well. Someone, somewhere noticed a "feeling" and they could get rid of the feeling by doing "response". (I need to poop, i should go poop. I feel horny, mating helps alleviate it) These connections between "feeling" and "response" stick around if they improve the likelyhood of reproduction and survival. This why we "know" what to do, but what about why we "feel" the things we act on? Because over thousands of thousands of years, the ideal age of any priority has come closer to an ideal point for survival. The young might prioritize playing to develop motor skills, and the brains are more receptive to all kinds of impressions to speed up the development of necessary skills. Focus shifts to "social" as you age, to learn how to and potentially find a suitable mate. This happens when one should be strong enough to survive on their own. "social" becomes "reproduction" when the reproductive system is ready. As you age and the reproductive system degrades, the priority might for example change to "care" where instead of making more babies, you ensure the survival of your genes by caring for children and grandchildren that hold your own genes (grandmother feeds grandchildren because theyre part of her)

Autonomous processes happen by themselves. Cells divide, guts digest, hearts pump.

Reflexes happen in the local nervous system. Eyes blink to protect themselves, hands jerk away from hot stoves, pupils contract or dilate.

Instincts are hardwired into the brain. Hungry - seek food. Horny - seek mate, tired - sleep.

Emotions regulate social behaviour. I like this individual - spend time with it. This is my child - nurture it. This individual wants to hurt me - run away (or kick its ass).

Intellect solves problems. I can't reach this thing, but if I grab a stick, the stick can reach it. That individual is able to eat dirty potatoes by first dipping them in water - imitate it. That animal sure feels scary, but if you look closely you can see it's all a charade - eat it.

Every level is more fine-grained, but also has less control. Your intellect can't control your heart muscle or your cell division, but it can invent a microsocope and figure out how cells divide.

Civilization is arguably yet another level. We can organize in order to solve problems that would be too complex for individual intellects, and the best individuals for a task can devote their lives to it rather than spending a lot of time ensuring their own individual survival.

All of these systems are available to humans. Other animals have them to varying degrees, and even individual becteria have the autonomous processes that keep them alive. Viruses arguably don't even have that, and they're also not alive as such.

The AI singularity is a hypothetical next level, where we've created artificial intelligences so clever that they themselves can design even more clever artificial intelligences without our help.

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