Secondly, to become an expert on a particular product does not require going on officially sanctioned courses. To become an expert on a particular software system of Linux you need to read the available documentation, and then apply it in the real world. Once someone has a certain level of expertise in Linux then like any other software system they can hire themselves out as consultants. There are many different formats for documentation for Linux. For instance, to look at the 'man page' for the sort command, you type man sort man -k keyword will search all 'man pages' for a keyword.
Documentation is key to understanding any product.
Even simple products with simple GUIs need some documentation. The complex and powerful systems that Linux provides often come with a LOT of documentation. Besides the provided online text files you can find many books on Linux topics. It is easy to write documentation on Linux products because much of it is 'open source'. Briefly, Open Source means that the 'source code' used to build the program in the first place is available for inspection.