A lot of their revenue likely comes from youtube ads. There's a lot of youtubers making a fortune that way. As in millions. There are youtube channels who have larger viewing audiences than major tv networks. That's why a lot of highschool and college kids can make a very comfortable living playing video games on youtube. Or at least there were until recently, not sure how well they're doing now, as advertisers started pulling their ads off youtube earlier this year due to some controversies and youtube started demonetizing scads of videos as a result. It all started with some drama surrounding one of YT's top gamer channels.

Reading comixology's faq section would answer that question for you. Most likely the comic creator keeps the copyright, unless you sell the rights to a larger publishing house. That's how most indie publishing sites work. Comixology is owned by amazon now so I'm pretty sure it works roughly the same as kindle books, which is that the creator keeps the rights.