Most people believe that it is the leaves of marijuana that cause the high. The leaf isn’t what gets you high. It also doesn’t need to be dried. People dry it simply because it makes it last longer and not get moldy. It’s perfectly fine to smoke fresh weed.
The part of marijuana plant that contains the most THC and that people smoke is not the leaf, it’s the flower. That yellow stuff that people call keef, that’s pollen. It’s also very high in THC.
As for how people discovered that it got you high, animals have been doing drugs forever. Nearly all animals like to get high. Primates eat fermented fruit, deer eat shrooms, cats sniff catnip, horses eat a weed that basically has the same effect on them as nicotine does on us, etc. Humans also stumbled upon a few psychoactive substances in our history. The fact that we can control fire added to that. It’s likely that someone first ate marijuana and realized that it made them feel funny, then they realized that the effect could be enhanced by burning it.