One of the first things anyone who is learning Fusion needs to learn is the color coding of the triangle inputs.
It seems very simple now (years later) but I do remember my first few runs in fusion not really “getting” the orange triangle vs the green triangle vs the blue triangle that are common to pretty much every node you work with. (some have extra triangles as well).
Orange is your background.
Green is your foreground
Blue is your mask
Take that information and think in terms of any other compositing or photo manipulation program you have ever worked with. At first, don’t think in terms of nodes, think in terms of layers. What is on the bottom, what is on the top, what cuts a hole, and at what point do I want it to cut the hole.
From there, fusion becomes much easier to understand as you start building up very complex trees.
Glenn