If we talk about some passive income sources then making online courses and sell them is the best way.
Read the full article to do so.
How to create an online course that sells?

Select a topic

If you have knowledge about anything then it is your topic or if you do not know anything then go to udemy,coursera and learn anything then you can sell.Remember always learning an on demand topic is always worth it.

Research

You have to select an on demand topic so that can be sold.Before you make the course research about its demand.

Create a blue print

Make a blue print before creating the course and think of exercises and modules.You may have a module on field tested strategies.Creating a blue print can help you the way you create your app and sell it.

Create the course

You can make the course yourself or you can hire someone to do so then you have to pay him/her for that.Your course should be a premium product and always remember people are paying for that so make the course amazing.

Platforms to sell

There are so many platforms available to sell your course such as udemy,skillshare,instagram and so on.Otherwise you can also make a website to sell it.

Upload it such a organised way so that people can easily understand it.

How to sell your course ?

Blog: If you have your own blog then you can publish it on your blog otherwise you can ask other bloggers to sell the course on their blogs in return you have to pay them.
Youtube:you can create a youtube channel about your course knowledge and sell the course on youtube.the more you get subscribers the more you get the chance to sell your course.And you can also contact a youtuber to sell your course.
Social media:social media is a platform where you can easily promote your course to sell and you can also join some groups specially on facebook you can find some active groups to sell your course.but remember do not die to sell your course you just have to advertise it and make others realize the worth of this course.
Email lists:if you have some good number of email lists then you can mail them to buy your course.you can also get email list if you have a blog.you can also buy emails from others.

Remember creating a course can not help you but doing its proper marketing does.So always put efforts to advertise for your course then only you csn generste some extra income from your course.Best of luck.