The Machine is the apparatus underneath the rest of the work: Nand to Tetris, networking, Linux and systems, databases and operations. A topic belongs here when its payoff is that something previously opaque stops being magic.
There is no syllabus and no order. A note appears once a layer has been understood well enough to depend on, and it keeps the evidence that earned that trust.
Learning to make a checker useful without quietly giving it authority over the services it checks.