nuts and bolts, the
The essential or basic aspects of something, as in They have lofty goals but don't specify the nuts and bolts of how to achieve them. This expression alludes to basic working components of machinery. [Mid-1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.