Building a binary package results in a
.deb file. This usually does not contain source code, as build artefacts are binary.
Exceptions to this is software written in Python, PHP, and other interpreted languages. Because it's interpreted, you can't build without restricting the end users to change the environment (e.g. Python version). And yes, there are workarounds for this, but this is not recommended. The Debian policy for Python for example is to compile it at installation time (Debian wiki: Python/Policy).
Feel free to inspect the contents of your package after building. Also see examples of open source software binary packages.
Edit - Using PPAs:
When uploading a package to your PPA, it will be a source package and built by Launchpad cloud servers to a binary package. Both will be published. There's no way build your packages locally and publish your binary-only packages to a PPA. You'll have to set up a repository yourself.