For selling merch like t-shirts and other clothing items here's a tutorial how to Create and build shirt press
Selling "limited" t-shirts with your brand name or slogans is another side hustle YouTube's do... They might claim they have 1000 available so get them now while they last..charge $20 for each shirt...they do this every few months for various YouTube channels...so think every season come out with a new shirt...sell 1000... $20,000 every 3 months...4 times a yr... Extra $80,000... Also having free giveaways of your t-shirts to your "subscribers" gets people to subscribe to your channel