For TikTok the RPM (revenue per 1000 views) is around $1 without taking into account brand sponsorships. If they post 30-40 videos a month they would need to average 100k views. That is hard to do, but not impossible. Even harder is to make money by selling crap from brand deals while keeping your audience engaged. Creating and producing that much good content while negotiating brand deals is a full time job.