the expected result shows only the second row is enclosed in squiggly brackets

No, that's not what it's showing. That's just an effect of the display. There is one overall List, and it contains 20 Columns. And each Column contains one List, and each of those Lists in turn contain three Integers.

Your code produces one Column that contains one List which in turn contains three Lists and each of those contains 20 Integers.

Okay, the section is on Tables, so I assume we want to use Table rather than threading or any other "fancy" stuff. The result should be a List of 20 Columns, so the general shape of the code will be something like this:

Table[Column[ ...], {i, ...}]

We can fill in the iterator bounds easily (it's 20):

Table[Column[ ...], {i, 20}]

Each Column should have a list like {n, n^2, n^3}. I used i for my iterator, so

Table[Column[{i, i^2, i^3}], {i, 20}]

For what it's worth, Column is kind of a weird thing to use here without any contextual justification. Not sure why they would expect you to use column, but it does make the problem a bit confusing, in my opinion.

So, this kind of complaint about Wolfram documentation is very common for people new to the language. I always find this amazing, because Wolfram documentation is literally the best of any I've every seen before. I mean literally, and by a huge margin. Of course, I haven't seen everything, so your mileage may vary.

The question is why do newcomers feel this way? I think that it's just a choice that Wolfram made to make documentation most usable "in the long run", i.e. they don't really target Mathematica novices. You'll only be a true novice for a short period of time if you stick with the language. At some point, you'll start seeing the patterns, you'll know what to expect, you'll know how to find connections, you'll be able to guess at options and variants, and you'll just start grokking the jargon. Right now you feel like you need better examples, but after a bit, you'll realize that the examples you used to want are completely superfluous. Right now you're probably thinking "why can't they just explain it in plain terms?", but after a bit you'll realize that the explanations are actually incredibly rich and use consistent patterns--they are in fact very clear if not "plain" to the uninitiated.

Yes, I tend to agree. I was trying to make a similar point. But I'm trying to put an optimistic spin on it. If you can get through the learning curve of a few weeks, your productivity will start on an exponential climb.

I'm not understanding the difference though I know there is one.