SceneTree (Godot)
Godot's main loop, and the tree of nodes that everything in a running game hangs from.
In Godot a scene is just a saved branch of nodes, and the running game is a single tree into which such branches are grafted and pruned. The tree is owned by SceneTree, the engine's default main-loop implementation: it drives _process and _physics_process, keeps track of groups, pausing and scene changes, and is reachable from any node through get_tree().
The ordering matters. _ready runs bottom-up, so children are set up before their parent, while processing runs top-down. queue_free deletes a node only at the end of the frame, which avoids anything pointing at a half-removed node mid-update. Talking upwards in the tree is best done with signals, a built-in take on the observer pattern.