Yes, especially for some unit testing is done like "atomic testing" (test the smallest possible part, if they could they would test individual lines), but turns

Yes, especially for some unit testing is done like "atomic testing" (test the smallest possible part, if they could they would test individual lines), but turns out that the putting together is another crucial part with often not less complexity and where the corner cases come in. In that sense even not much like the unit vs integration testing difference & debates, because that line is blurry and just depends on your definition of an unit: It may be the single heavy algorithmic function, it may be the whole state machine, it may even be two heavily dependent and cooperating tasks in an embedded RTOS... and you always know you are in trouble if people discuss more what an unit test is and not is (instead of the value whatever thing provides). I "love" btw those code bases rich with tests but 99% of the time not finding a bug, but breaking on the tiniest detail change.. not supporting refactoring like it should be but making it slow and everyone wanting not to refactor because of the crap test rat tail.