not all the business needs much investment - when COVID hit - many people started their side hustle from their home and they earned so much with zero investment
The side hustle needs dedication and that is the only thing which can make a business successful
There's affiliate marketing which is already known. You don't have to invest in stocks and products. The logic and simplest explanation of it is that you'll be promoting others' products and you need to use their affiliate link for you to have it count when someone clicks and buys the product you endorsed.
This is a side hustle but if you're able to make it your main niche and focus on the way of marketing it. You'll be able to earn a lot of passive income from it especially if you've made an interesting article/blog or video introducing the product and it gets viral.