With zoom lenses, the aperture doesn't physically change when you zoom in. Meaning, the window at the centre of your aperture ring (closed) stays the same.
The reason why it says that f5.6 is the maximum that you can stop it down to aperture wise, is not because the aperture actually gets smaller, but due to the extended focal length, the lens is physically letting less light in.
Consider your nifty fifty at f1.8. If you slapped a big tube on the end of your lens. It would obviously let less light in than if there wasn't a tube. So hypothetically, you could say that the aperture figure has increased.
You will notice this on lenses where you can manually set the aperture on a ring (much like a focus or zoom ring) to a set aperture.
For example:
I have a 28-200mm f1:3.8-5.6 lens. If I set the aperture to f3.8 manually and then zoom in (so that the aperture is locked). I am still going to get a photo with an aperture of 5.6.
The good lenses are manufactured in such a way that they allow enough physical light into the lens even at full zoom to maintain a constant aperture. Basically, that there is no loss of light by zooming.
Now if you think of that on a scientific / technical level, you will understand why those lenses are so expensive.
lastly, Your kit lenses aren't bad quality because of their focal lengths or aperture numbers. They are of entry level quality because they are mass produced.
You could still end up paying a lot of money for a good quality lens with the exact same specifications (i.e. 18-55mm f1:3.5-5.6). The difference isn't in what it does, but how well it does it, if you get me.
As for how long it takes to understand all these things. For me I am still learning and i've been doing photography for years. I say, just have fun and don't worry too much.
Your kit lenses will give you great photos, but if you can afford to invest in a nifty fifty, do it. You'll have a lot of fun.
Good luck!
