Watch
https://www.youtube.com/watch?v=SnpAAX9CkIc Rise of the Systemic Game, Game Maker’s Toolkit
Read
https://www.gamedeveloper.com/design/a-guide-to-systems-based-game-development
https://www.gamedeveloper.com/design/design-pillars-the-core-of-your-game
Tools to look at:
mini metro
Look for combat systems, economy systems etc.
Game loops are core for your game system, they explain how the game will work mechanic and flow wise, simplifying your game into small flow charts.