You need to be actively coming up with the core gameplay loop, thinking about progression and mechanics/systems, creating the UI/UX (User Interface/User Experience), and building content that makes use of all that, those are the elements of game design and the role of a game designer.

https://www.gamasutra.com/blogs/JoshBycer/20210609/382911/Why_The_Science_of_Game_DesignMatters.php