How did she understand all that with just a sentence ? That she dressed like a man and all...

It's the word restraint that triggers it

Yeh I got that. They explained that she read the old version of a book. But I didn't understand how she found out that she was a commoner, who dressed like a man, like it was logic

Maybe that old version of the book is only available in public libraries where men are the only ones who can access it because of their gender. Old traditions I presume, especially in the old era