I was wondering which of the following is correct:

1) The primary focus of the essay is to explore the extent to which learners’ errors are interlingual and intralingual.

2) The primary focus of the essay is to explore to what extent learners' errors are interlingual and intralingual.

One more question


Very often if we are making the question phrase "to what extent" into a relative clause like structure "the extent to..."
we have to say the extent to WHICH? but not what?

I'm very interested to know why and how the whole phrase
"the extent to which" actually work. I mean, I've seen relative pronoun {e.g. which}, preposition + relative pronoun {in which, on which...} but never a Noun phrase + preposition + relative pronoun. How does it really work?

Thank you so much