While we are discussing the Jenkins file structure, it is useful to note an impedance mismatch that often occurs between GUI-based tools, such as Jenkins, and the DevOps axiom that infrastructure should be described as code.
One way to understand this problem is that while Jenkins job descriptors are text file-based, these text files are not the primary interface for changing the job descriptors. The web interface is the primary interface. This is both a strength and a weakness.
It is easy to create ad-hoc solutions on top of existing builds with Jenkins. You don't need to be intimately familiar with Jenkins to do useful work.
On the other hand, the out-of-the-box experience of Jenkins lacks many features that we are used to from the world of programming. Basic features such as inheritance and even function definitions take some effort...