What is the most popular chess app for android that is used for playing against other Human opponents?
Every good chess website has its own app. Such as chess.com, lichess, chess24, ... Just lunch play store and enter your favorite chess site name, you will find its app. They usually have human mode or computer mode with various time controls. With chess.com app you are also able to challenge particular opponent by his user ID and in lichess it's even easier, create a challenge and send a link of challenge to your opponent.
It turns out some people here cannot get what is the most popular app to play against other humans in android even w.r.t my answer that I put some of those apps in order. It's clear that app is chess.com app, as the number of members of chess.com is way more than any other website and if someone still says this is not related to app, then 1M+ downloads of this app on play store compare to 10K+ downloads of other apps clear's everything.