Multiple 'apps' and order of uniqueness & possible override?

Wondering about the situation where say an Android project has 3 separate ‘apps’, that is, three subdirs with their own build.gradle, a, b, c.

  1. Do all the individual compile calls have to be globally unique for the total project?
  2. In the case that b depends on a, what if both of them depend on a compile. If a has it, does that mean that b doesn’t need to list it? What if b does list it and it is different? Will b’s override the original one in a? And if so, for both or just b?