Don't know how to pay in the Playo event app

I participated in an event app during the Playo event. It's a merge-type app. Basically, you connect the same numbers, for example, from bottom to top, or from top to bottom, and as long as they connect, the numbers get bigger. It's a fun app that even those who aren't good at merge games can enjoy casually. For instance, if you connect 10, a 10 combo display appears. Then, there's a mission to 'get coins by paying,' but the app doesn't show any payment options. I'm not sure, but maybe they want me to use up my diamonds? It's a nice app with no pressure to pay, which is great. Is this kind of app ethical and pleasant to use, perhaps operating mainly on ad revenue?