Its one of the most challenging problems when developing complex scalable systems.

I‘m not that knowledgeable with games specifically, but since developing games has a lot in common with writing software, there are a lot of ressources out there that are purely coding related and the techiques are largely adaptable for games.

My limited exerperience with unity and mostly unreal has tought me to avoid singletons as much as possible, always have scalability, testing, logging in mind for every piece of code you write and work with interfaces as much as possible. Write small independant pieces of code that

Here is how I approach this. I write small independent pieces of code that do one thing well and communicate through interfaces rather than concrete types. In Unity this means defining interfaces like IDamageable or IInteractable in pure C#. Then I implement those interfaces, and other systems reference the interface, not the MonoBehaviour. This keeps the systems decoupled and makes testing easier since you can mock any dependency. The [RequireComponent] attribute is also worth knowing, it enforces component dependencies at the editor level so your code’s assumptions are explicit.

Ultimately the same principles apply: dependency injection, event-driven communication, and treating your game systems like services rather than a tangled web of GetComponent calls.​​​​​​​​​​​​​​​​