These methods are aside from buying as many properties as possible to earn money over time passively, aside from buying that money DLC, aside from completing the missions that have bigger cash rewards, and certain other things. Source means stuff like which activity or diversion, the difficulty level if it's an activity, or the vehicle and location if it's a barn-storm.
I was initially emphasizing the fonts of key aspects of each method, but there are too many, so instead I'll simply tell you what to look out for: source and cash per unit time.
