Many Open Source projects provide their source code in different version, for the various systems such as Windows, Debian, Fedora etc. Since Ubuntu is Debian based, should downloading the project for Debian suffice and I won't have to do any modification to it?
One example I would like to give is Code::Blocks, they have a project source file for Debian and none for Ubuntu. So this Debian version can be downloaded and can be easily built in Ubuntu, right?
Yes, you can generally build packages that are made for Debian on Ubuntu, most packages in Ubuntu are like this.
In this example, you'll find that Code::Blocks is already in the universe component of the Ubuntu repositories, the package name is codeblocks