How does make solve the problem of only building what has changed in the current PR? It seems like this would only work if you have a single CI server and all o

How does make solve the problem of only building what has changed in the current PR? It seems like this would only work if you have a single CI server and all of your artifacts live in the filesystem on that server, no? Solving that seems sufficiently difficult that saying "make solves the problem" seems disingenuous, but maybe it's easier than it appears to me?