On large team projects, I've needed to manually edit the Ant Buildfile. On small programs I've written from scratch, I haven't needed to make or edit any sort of helper files for the build (e.g. never written a Buildfile for Hello World"). So my questions are:
No project built with Ant will ever build automatically without creating a build file.
Ant doesn't do anything without a build file.
Maybe your IDE generates one for you, or maybe you're not even using Ant to build your small project.