If Hermione's parents from the Harry Potter film series are Muggles how did she know about Hogwarts in the first place? How come her parents didn't wonder about her school?
If Hermione's parents are Muggles how did she know about Hogwarts in the first place? Someone from Hogwarts came and explained to her and her family(as they do to all muggle borns)
How come her parents didn't wonder about her school? They wilfully admitted Hermione into Hogwarts after they were intimated of it. Nowhere in the books or in the films, its shown that they had some kind of issues with Hogwarts or that they didn't know about Hogwarts.
Why she got a letter in the first place ?
There is this concept called the 'Quill of Acceptance and the Book of Admittance' described in Pottermore(offical site of JKRowling, the author) which states:
At the precise moment that a child first exhibits signs of magic, the Quill, which is believed to have been taken from an Augurey, floats up out of its inkpot and attempts to inscribe the name of that child upon the pages of the Book. No child has ever been admitted whose name has not first been inscribed on the book’s yellowing pages
This is how Hermione would have been enrolled and admitted into hogwarts. So when a child whose name is in the book come of age(11 years old), they will get their letter
How she got her letter :
Its never mentioned in the book or in the movies how Hermione gets her Hogwarts letter.
Hermione about her admit into Hogwarts in the books:
“Nobody in my family’s magic at all, it was ever such a surprise when I got my letter, but I was ever so pleased, of course, I mean, it’s the very best school of witchcraft there is, I’ve heard –"
-Harry Potter & the Philosophers Stone
To every muggle-born witch/wizard who needs to be given the Hogwarts acceptance letter, a teacher from Hogwarts goes to the kid's parents and explains to the family and the kid about the letter, Hogwarts and the magical world.
Two examples of this:
One -- Snape tells lily of how she will get her letter
“And will it really come by owl?” Lily whispered. “Normally,” said Snape. “But you’re Muggle-born, so someone from the school will have to come and explain to your parents.”
-Harry Potter & the Deathly Hallows
Two -- Albus Dumbledore goes to Tom Riddle to give him his acceptance letter and to explain about magic and Hogwarts.(Harry Potter and the half Blood Prince)
From the above, we can safely assume that one of the teachers from Hogwarts gave Hermione her letter and explained to her parents about the magical world
Probably the same way that Harry Potter's mother was informed since she was Muggle born like Hermione. I guess someone from Hogwarts was sent to the Grainger house to explain about the school. Hagrid was sent to where the Dursley's were hiding, when Harry did not respond to the letters.