I've been focused on this topic a lot myself lately - so much so that I've literally just been making videos on this very question. I think it can help you get some ideas.
YouTube is a brilliant method if you can put in the work upfront, but ideally you would mix in some other passive income methods with your youtube channel to get the best results. Feel free to PM if you have any questions.
10 Of The Best Passive Income Methods (that are proven to work):
Of course, there are many ways. For instance, if you don't want to invest or do affiliated marketing, you can do something like this:
step 1 - register with Printful
step 2 - design a logo or a sign with will go on t-shirts
step 3 - set up and integrate your Printful with an Etsy store
If customers on Etsy like your logo and buy one of the t-shirts you will get paid.