How does Make decide what to rebuild?

Make works from a dependency graph defined by rules, each of which has a target, its prerequisites, and a recipe to build it. To decide whether to run a rule, Make compares file modification timestamps: it rebuilds the target if the target doesn't exist or if any of its prerequisites is newer than the target. So if you edit one source file, its timestamp updates, making its object file out of date, so Make recompiles just that object and then anything downstream that depends on it (like relinking the executable), while untouched files, whose objects are still newer than their sources, are skipped. That timestamp comparison is exactly what makes builds incremental: you don't recompile the whole project for a one-line change. The correctness of this depends entirely on the dependency graph being complete, if a target's prerequisites don't list everything it actually depends on (notably headers), Make won't know to rebuild it when those change, producing a stale build. So "what to rebuild" is "anything whose recorded prerequisites are newer than it," which is only as reliable as the declared dependencies.