Passive income would be stocks, bonds, crypto and other investments. If you are talking about putting in the work and then that work making you money over time, (what many people mistakenly call passive income) I would say writing books, creating a very informative blog and having an ad network place ads on it.
The amount of money you make is directly proportional to the amount of work you put in up front.
Don't fall for the big red easy buttons people will try to sell you promising passive income. Everything takes works. Including researching the real passive incomes I mentioned above.
Nothing is really, truly passive, as it all takes effort to setup and maintain to keep the income at the level you require.
For me, the closest thing is my rental property, but online it's affiliate marketing. However, that still takes a serious initial effort before you can "set it and (almost) forget it". I still make sales from initial setups I made 15 years ago, but it gets less over time if you don't "dust it off" once in awhile.
Investments in residential and commercial real estate, with subsequent leasing.
Purchase of a visited website, a popular community on a social network (requires regular updates and administration, or you can hire a manager who will monitor the project)
Occasionally you can get passive income from a large number of referrals in affiliate programs
Try PicturePunches. It's a memes site that you can earn from. You can refer people and earn passive income. Also you can post memes and earn money. It's probably the easiest way to make money online, period.