If you really want to learn how to design game systems, I recommend starting from board games.
System designers are rare and in high demand in the industry. Especially with the advent of new business models and technologies, people capable of breaking down a game into systems and finding the connections between them are really rare and highly appreciated.
The temptation when starting to design systems is to start by opening a spreadsheet. The spreadsheet is by far the most used tool for game development. However, there is no need to open it from the start. When you start thinking about breaking a game down, it is best to use all your senses. Better to use touch and sight, above all.
The best way to start thinking in a system is therefore to prototype the system in the form of a board game. When you work as a game designer your job is to study games that already exist. Also to carry out this very important task, I recommend that you synthesize the video game in question (or the more interesting parts of it) into a board game to really understand all its parts.
Creating board games could save you months of development and playtesting!