The reason for this is that nobody took the time to write a proper background document on Github Actions. The kind of information that you or I might convey if asked to explain it at the whiteboard to junior hires, or senior management.

This syndrome is very common these days. Things are explained differentially: it's like Circle CI but in the GH repo. Well that's no use if the audience wasn't around when Circle CI was first new and readily explained (It's like Jenkins but in the cloud...).