Question
What do the terms upstream and downstream mean in I.T.?
Answer
If a project B is downstream from project A, project B has evolved from project A. In the genealogy of software/code/projects, project A is said to be upstream to project B if project A is an ancestor of project B. Project D is said to be downstream to project C if project D is a descendant of project B.
For a detailed explanation, see https://reflectoring.io/upstream-downstream/