In Interstellar, how did Cooper get into that 5 Dimensional structure? Is it because of Gargantua (black hole)? Or is it because of 'they'? Or any other reasons.
I didn't understand that part well enough.
It was because of "them".
What we can assume is, that this weird bookshelf world is not what it actually looks like inside a real black hole. In fact it doesn't "look" like anything, it's a plain singularity, which there is no escape from. So "they", the "Bulk Beings" -- which are basically 5-dimensional superhumans from the future -- transported Cooper into this tesseract room, which they built in exactly this way for him to have an intuitive spatial way to navigate time and send his message back in time to his daughter.
Without the intervention of "them" Cooper would simply have gone lost in the singularity of the black hole. What still stays unclear is, if that black hole singularity was necessary for him to be transported into this tesseract, i.e. if he needed to get into the singularity in order to reach 5-dimensional space in the first place, or if this just was a convenient moment to "get hold" of him, or if "they" even planted the tesseract somehow at the singularity.
But it was not Gargantua alone that suddenly set him into this tesseract room. That room was a conciously constructed (by "them") device to facilitate his time-defying communication with his daughter and "somebody" put him there deliberately.