A small amount of Google AdSense (<$100 monthly.) Sortable makes up the vast majority - https://sortable.com/ . Switching from only-AdSense to almost-entirely-Sortable is what made me able to afford to live off the site's revenue: my earnings doubled overnight.
I believe they do header bidding for their own demand, but you can squeeze more revenue out of your traffic if you learn to manage your own headerbidding stack. Take a look at prebid.org and checkout the r/adops reddit. You might find some inspiration there.
