and how do you test a system with tens of components each runs in its own service with its own pace its own failures? for some reasons people treat testing just

and how do you test a system with tens of components each runs in its own service with its own pace its own failures? for some reasons people treat testing just as unit test, i don't get how you test the whole big thing as a whole. in addition you never get the same hardware nor the same software, network management, same number of servers/services as you have in production. This is the real challenge, unit testing is piece of cake and I never skip it, how to test the whole system this is where I get confused (without getting test that run for hours which are unusable and overly complex).