It usually comes out of a need to do something that an existing (open source) framework natively doesn’t so you build upon it organically.

For instance, I built upon (customized) an open source framework by initially starting a test suite creation for a project. I wrote the suite with pre and post test case functions and added functionality to the suite that continuously polls device resource utilizations in the background. I take min, max, and average device utilizations from the data collected to factor into the pass/fall case decisions. I’m currently in the infancy stages of turning all of the poll statistics into graphical output instead. So overtime, 1 project turned into customizations of the original open source framework and I’m taking it into another direction like automation + test monitoring.