It has blocked a total of 2.337 Million network requests in the last 3 years for me. That is an astonishing no. of ads and trackers. The 21% here means that for every 5 network requests made on a webpage I opened, roughly one of them was for an ad or a tracker. 1 in 5!!!
You can use uBlock on android via firefox. It allows some plugins and uBlock is one of them. Also dark reader is available as well.