There are dozens of ways to generate passive income. However, the option you select has to do with two metrics: time and money. Either you have a lot of time or a lot of money. Most people usually don't have both. But, if you have a lot of money, generating passive income almost instantly is easy. You can buy up some real estate and begin enjoying rental income. Or, you can invest in a dividend fund or some other investment vehicle that will begin generating a steady income for you.
I have only dabbled in drop-shipping before when I had an eCommerce platform 6 years ago or so. I think it is something that you could do on the side, but you would want to do in depth research on the industry you want to get into before setting up shop. It may be a little less passive up front, but over time you could take your hands off the wheel.