They've delegated, automated, streamlined, systematized, etc. Not with the intention of sitting on some beach somewhere for the rest of their lives and watching the checks roll in, but with the intention of freeing up their time to create even more value that they're inspired to create, either by leading that business to the next level of greatness and service to greater audiences, or by starting a new business.
Another great way to get started is to identify an area of interest you have. For instance, Robert Duff has been successful in building passive income by selling books on Amazon. Then, go out and start talking to people. Ask them, “What are you struggling with right now? What are your biggest pains? What’s something you wish existed that doesn’t?” That’ll give you some ideas about where to get started.
Ebooks are one of my favorite sources of passive income. Now, you can do this the simple way and just publish it on Amazon's KDP. Or, you can go all out and build yourself a book funnel. Book funnels are powerful, but they won't be fully passive. For example, if you do a free-plus-shipping offer for your ebook (converting it into a physical book), you'll need to create some one-time offers (i.e. extra training) and up-sells (i.e. an audiobook). But, a book funnel can be very powerful.
Revenue sharing sites give you the opportunity to earn money online without having a website or a product to sell. If writing is your hobby, for example, then you can earn extra income by writing high quality articles and submit them to different locations on revenue sharing sites. I must admit I’ve never tried it, but I wanted to get it out as it can be a viable option to get started with.