Louis Rossmann in his videos has complained about there being no authorized way to obtain technical repair documents. But he has also said that he is lucky enough to have people (some of whom apparently work or worked at Apple) that have given him troves of such documents. I couldn't find the video where he said that again, but I did find one earlier discussion on here about it[2]

[1] https://www.youtube.com/user/rossmanngroup/videos

[2] https://news.ycombinator.com/item?id=16832323