How to design a game app

Learn a coding language, Find a game engine that supports it. Learn the basics of the Engine. Maybe try learning modeling if the game is 3d or animating. It's your choice.

You cant just show someone how to make a game, its like telling someone to speak english when they dont speak english, you need to learn the programming language and make the game, like learning english

Show me how to make the game

For example you can use WebGL in web, Unity and Unreal on different platforms and also threejs on web for efficient game development. Check threejs sample code on any website