It's called a Master Document. It has links to all its sub-documents.
The feature was first developed by Word Perfect. MS put it into Word so they could say it had the same feature, but they never got it to work.
I spend some 20-odd years supporting, amongst other things, MS Word on Mac and Windows. I can assure you that if a document has many edits it can become very unstable, to the point of being unrecoverable. When working on a long document I would do a Save As to a different version number each day. This puts all the edits in order and makes the current file more stable.
Because of this, I did try MS Word's Master document feature because it would let me deal with smaller document files and found it a rapid path to madness.
Read this site --
https://addbalance.com/word/masterdocuments.htm
Master Documents work well in LibreOffice, so well that they develop and distribute their documentation in Master Document format.