To "bring it all together" is also an idiom. https://idioms.thefreedictionary.com/bring+all+together It is most often used to mean to successfully plan an event like for example a wedding or funeral or party etc and it all works as planned. You would have bought it all together. In this context the idiom is not being used as an idiom, it means for the researcher to "find out" and successfully get the results they wanted, they had to read thousand of documents and interview hundreds of people, "to bring it all together" to achieve the results "find out" what they wanted to know. The other answers are correct but you will confuse yourself unnecessarily for this context if you think too much about the grammar aspects.

I think because the speaker is talking about their data, not about the people or scientific fields they gathered it from. The data can be considered one single inanimate object, so you can refer to it as 'it'.

No, 'it' is often used to refer to a collection of things that are being unified in some way. (THINGS meaning ideas or objects, NOT people or animals). usually used with "put" or "bring": "New speakers, a guitar, two keyboards, put it all together and you've got a nice little recording studio." "Them" could be used, and it is technically correct, but it isn't usually the word we use.

"It" in this case refers to the collection of ideas as a whole. There's one collection, so it gets "it." You can also say them, in which case you're referring to the various concepts individually, rather than as a collection. The overall meaning of the sentence wouldn't change in that case. The purpose of using it here rather than them is it makes the speaker sound more sophisticated, and it also signals a transition in the talk from focusing on the small details to now looking at "the big picture." Merlyn is right, you wouldn't use "it" for a group of animals or people, it would sound a bit degrading in that case.