I've been thinking about how to earn passive income for a while - I believe that earning money should be separate from the time I invest in it. Here are some of the strategies I've been looking at and thinking of employing, with specific details to help you out. Written with examples for the Singapore context, but the ideas apply anywhere.
This way of creating income is just as it says - create something that others can consume over and over again. These require upfront effort but could pay in the future. There are two main ways to do it - either create content, or create products (like a website or manufactured items). These can scale without your active involvement.
However, these come with different risks than investing or real estate - you'd want to invest more time into thinking about what you want to create. It's not as easy. Also, these require more time and effort than the other two methods; the passive part comes from the scalability, which may or may not ever come.
This would be the YouTube stars and yours truly (haha!). Creating media in any form - comics, videos, podcasts, writing, courses, 3D models - and distributing it to a wide audience.
For apps, you could charge monthly subscription services, or a commission for using the product. Examples are Stripe and Netflix. And also, Ghost. Software products tend to be the easiest to scale, but if you could mass-manufacture something for a long time, physical products could be worthwhile.
Monetising your creation
These methods would help you earn money from what you've created. However, a lot of these require scale to work - I've heard that for creators, you should be spending about half your time marketing your work and the other half creating.
This could be a good way to get cash initially to run your project, though you'd often have to give away something else in return. For software products it tends to be equity; otherwise, it could be "vanity" items such as being the first to support X product, or exclusive perks for believing in the project.
You can include links to products and earn a small commission from them when people buy through your link.
People would need to pay to use your work, or in the case of books, you get a cut of each book sold.
You could have exclusive content or features limited to only paying customers.
You could put ads on your site.
If people find your app/content useful, you can encourage them to donate.
Creating related products
You could start a brand/franchise and create different variations on the same theme. For example, the Marvel Universe includes movies and comics and toys too, or adding features to your app.