When you publish a blog post, it is getting indexed by search engines which will keep sending you traffic over time. You won’t have to do anything to get this additional traffic, but it will keep compounding. Some users will sign up to your mailing list based on this older content without you having to do anything. The more posts you publish, the more visitors you will get. Same with a good answer on stackoverflow that keeps getting upvotes or with a helpful open source project on github that keeps getting stars.
A similar thing happens with apps you build. Once you have some paying users and your app is indexed by search engines, you will keep getting paid and get new users with a much lower level of effort on your part. You will probably need to maintain and update it but you will not have to invest the same level of effort of writing the app in the first place.
This is the big advantage of creating content and products, you get benefits from what you create even after you are done creating it. You can move on to create other things.
One place where it doesn’t usually happen is your work place. You get paid for the exact number of hours you worked - nothing you built will keep paying you money after you’re done working on it, even if it makes the customers of your company a lot of money over time. Everything you work on is private - no matter how well you did your job, only you, your boss and coworkers will know about it.
If you are working on something in your free time, make sure you have something you can show at the end of the week. Little by little, you will start seeing results.