How do Neurotransmitters get into the neurons?
The general rule (though like much of biology there are exceptions) is that each neuron releases a specific neurotransmitter: neuron types are often named by this principal neurotransmitter plus the suffix -ergic: a cholinergic neuron releases acetylcholine.

There are also similar molecules that do in fact act throughout the body, but we don't call these neurotransmitters, we call them hormones. Sometimes they are exactly the same molecule: epinephrine, also known as adrenaline, is both a neurotransmitter and a hormone; it can act as both because different receptors have different affinities, and the circulating hormone levels are typically much lower concentration than the amount that can be released in a more confined space.

Additionally, neurons have receptors for different neurotransmitters, so for example dopamine does not affect all neurons, only neurons that have dopamine receptors. Different cell types can also have different types of dopamine receptors, and those receptors can do different things to the cell. The same neurotransmitter could be excitatory to one cell and inhibitory to another.

Broad simplifications of nervous function such as:


  Dopamine is also known to increase our feelings of lust


really don't say anything broadly about the function of dopamine. I could similarly say that computers control airplanes: that statement is true even though you will fine airplanes not controlled by computers and because you will find the vast majority of computers don't control airplanes. However, if I turned off all the computers in the world, a lot of airplanes would have their function impacted.

How do neurotransmitters get into neurons?

Most neurotransmitters are synthesized within neurons from other ingredients. GABA, for example, is derived from an amino acid glutamate. Glutamate itself is also a neurotransmitter. All neurons that release neurotransmitters have pumps that concentrate these neurotransmitters into vesicles. Neurons often also have transporters to pull released neurotransmitter back into the cell so it can be reused, for example a common class of psychoactive drugs are the serotonin reuptake inhibitors: these drugs inhibit the protein that pumps serotonin back into serotonin-releasing cells.

General statement

Use of knowledge from neuroscience to improve AI algorithms has moved far beyond what a novice can contribute to. If you want to utilize biology in AI algorithms, you have to be prepared to become an expert in both. I'd suggest starting with a basic neuroscience textbook if you want to learn about neuroscience, for example books by Purves' Neuroscience or Kandell's Principles of Neural Science.