The very short answer: they do it just the same way that you learned all that difficult grammar in your own mother tongue.
Slightly longer:
How does anyone learn their native language?
Well, according to linguists, they do not.
The process of getting to know how to speak your mother tongue is called acquisition, not learning.
Learning is what other people do when they learn the language as a second (or third, etc) language. They learn the rules, the grammar, and they try to apply them.
Young children subconsciously analyse the words and phrases they hear around them, and they build up a "grammar" in their head, as well as a dictionary, a thesaurus, enormous lists of collocations, etc. They don't actually sit down to study, they just absorb and analyse.
So a native speaker knows that he should say he works instead of he work, and even when he encounters a verb that he has never seen before, he knows the third person singular will get an -s in the simple present.
The amazing thing is that he knows that, even if maybe he does not know anything about grammar. Even if he doesn't know what a "verb" is, what "simple present is", or "third person singular". Even if he has never seen a grammar book, he will be able to correctly say he (new verb)s.
Just look around you at any young kids. You may think that your mother tongue is easy, because even little children speak it very well. But it's not. It's as easy or difficult as English is, and those kids (and you, when you were little) are doing what children everywhere do: they build up an understanding of the grammar of their mother tongue without even realizing it.
Ask a five-year old why he just conjugated that verb correctly, ask him in which grammar book he saw the rules for that, and at best you will get a blank stare.
You had discovered an enormous amount of grammar by the age of three, just like any kid, in any language.
This question reminds me of this old joke:
I am so glad I was not born in China, because I don't speak a word of Chinese!
Best Answer
The best answer is in. People have an interest in things. See the attached graph. It includes both the simple present (interest) and simple past (interested) forms. The number of uses of interest(ed) about things is so low that it is represented by just a line straight across the bottom of the graph. I did not include interest to because in your sentence that is not grammatical.
Examples include:
1 I am interested in human rights.
2 My interest in human rights began with...
3 He is interested in that woman...
If you want to use about, a common word used with it is curious or curiosity.
Examples include:
1 I am curious about human rights.
2 My curiosity about human rights began with...
3 He is curious about that woman...