https://www.theverge.com/24100464/altstore-pal-dma-eu-launch-delta-nintendo-emulator-clip-clipboard-manager

TL;DR The creator of AltStore has two of his own apps within the store but people can add "sources" (repositories of apps made by other people) and download them through AltStore. So instead of paying for each app, you only have to pay once and then add the source if the app you want is available through this method.

AltStore existed before this. I used it for over a year to get a modified version of YouTube without ads and automatically skip over sponsored sections. I've also disabled shorts and other annoyances.