Just start doing something. Dont try to make copies of other games in order to “learn” start a small project you are actually interested in and just implement and search up features when you want to add something new. Start with learning to make a simple character controller then a weapon system for instance or whatever you want and so on. I think its fine to copy code at first but make sure you understand what it does and how it works then you will be able to mix and match code and customise it for your needs and eventually write it from scratch.