This directory contains targets which represent the various GN arguments used to alter the build graph. Instead of directly using a given GN argument in a build file, add a dependency on the matching target defined in this directory.
The board, product, and dev subdirectories host targets for GN arguments declared respectively in //build/board.gni, //build/product.gni, and //build/dev.gni. In addition, the build file in the root directory contains targets which aggregate targets from subdirectories.
This is most obvious when using tools such as gn path which will show the targets in the output paths instead of jumping from the target that needs the GN argument straight to the labels listed in that argument.
Compare:
//i/am:foo --[private]--> //this/is:bar
to:
//i/am:foo --[private]--> //build/input/board:bootfs_deps --[public]--> //this/is:bar
The latter output makes it very clear how //this/is:bar is introduced in the build graph.
The targets defined in the root directory may combine targets from multiple subdirectories. This level of indirection makes it easier to introduce or remove GN arguments without having to modifying all call sites.
For example, an aggregation target will expose all dependencies for the main zbi; currently these dependencies are sourced from GN arguments from the board and the product files, but in the future we may add an argument to let developers manually add to the zbi. Thanks to the aggregation target, this change would be invisible to the targets actually declaring the zbi itself.