(For whomever wants a primer)
Originally dance tutorial videos. Dance videos are popular on TikTok as a social meme for young people. Short videos showing a certain dance step set to random song. Tutorials helped other young people do the dances themselves, so they became highly trafficked by people who wanted to be cool (basically everyone in high school at the time). These videos are also pushed by algorithms because well-dressed, fit, young, attractive females get views from essentially every gender and age group (a story as old as time).
At some point, the d'Amelios joined up with some top LA TikTokers and formed a TikTok collective called Hype House. This is what separated them from other popular dance TikTokers (alongside general personality, charisma, fashion sense, etc.). At that point (and really before) they had transitioned into social and family dynamics as a dominant form of content. Essentially celebrity culture. Who were they dating, wearing, seen with, what were they doing, etc. The Hype House was a rented LA mansion where they all independently filmed their videos, but most didn't live there. Think Studio 54 meets Def Jam or something, but with teenage drama. They all raised their own individual fame by capitalizing on the fame of each other. Despite being massively popular for a while, at some point, it became lame to be part of the Hype House, so the d'Amelio's left and continued doing their own thing.
Similar to the Kardashians, their parents were business savvy and capitalized and expanded their reach and exposure in smart ways. Their income comes from posting sponsored videos, but also from actual modeling and ad partnerships (they've been in super bowl commercials, etc.) and now a reality show on Hulu where they have a chance to capitalize on the end of the Kardashians show to be the "it" celebrity family.