When one says ''the nuts and bolts of something' they mean 'the basic facts or practical things that need to be understood or done'. For example
He was a genius on paper, but when it came to the nuts and bolts of running a business he was a failure.
I am thinking about
C'était un génie en théorie, mais quand il s'agissait des rudiments d'une entreprise, il était un échec.
but I guess there are better ways to express the idea.