That's a lot of questions lol.

So, the current model of addiction leans towards neurotransmitters being the triggering factor.

That means that whatever a given addiction is to sets off an interaction with one of the chemicals that make the brain go "WOW!". Exactly which one varies by drug and activity.

As an example, cocaine works with dopamine as the primary neurotransmitter. It causes a lot of it to release at once, and you get high.

All of the addictive drugs do something similar, it's all about which ones set off which NT/s.

The big ones that get that job done are dopamine, serotonin, norepinephrine, oxytocin, and the endorphins (there's multiple types). There's other factors in getting high, but the shit that will dig into your brain and take hold in addiction tend to either trigger those to release (singly or multiply) or bind to receptors for them.

However, that's not the only thing that causes addiction. Addiction is a more complicated disease than can explained away as only chemicals. The person's psychological history, social factors, genetics, and state of mind during use contribute strongly to the process.

There's other chemicals we get from external sources that do the same things, but they tend to have a much lower risk of triggering an addiction by themselves. Weed is an example of that. The addiction threshold is much higher than most recreational drugs, and that's in part because the receptors they bind to don't have as strong an effect.

But! The there's that connects all addiction is that at some point, neurotransmitters are involved. That's why you can get addicted to things other than drugs. Sex sets off a release of all of those aforementioned big NTs. Thrill seekers tend to get hooked on the norepinephrine. Gambling hits dopamine the hardest.

The key factor is that if something makes you feel good, our brains are hardwired to seek that feeling. The more intense the good feeling, the higher the chance of addiction being formed.

And, yes, the reason different drugs give different highs is that the exact chemicals involved vary, and the proportion of them varies as well. Something that's more dopamine focused is going to be a more energetic experience, whereas endorphins tend to come with a more relaxed and euphoric vibe.

I gotta give my usual disclaimer though. Other than a quick search to make sure of spellings , I'm pulling all this out of memory. And I'm fucking old, so I never trust my memory 100%. I'm also not willing to write a book here, so I'm leaving out a shit ton of detail in favor of hitting the questions you asked in total. Plus, addiction isn't high (hah) on my list of continuing education as a hobby, so my shit is likely out of date. That may or may not mean it's wrong, just that it isn't cutting edge info. So, grains of salt needed (though hopefully not "bath salts").

Fwiw, I will add that there's a misconception about addiction, that there's a difference between "physical" and "psychological" addiction. There isn't really. It's all tied together, it's just that some chemicals do have an extra oomph because they give a much bigger boost to the NTs involved. There's also a misconception about specific drugs not being addictive just because they aren't as fast to trigger it, and the factors like one's mental state and history have a bigger influence. But they're still addictive, just lower risk