How does the hierarchy of procedures and objects work?

How does the hierarchy of procedures and objects work? Who runs first, what sequence?

Nothing runs outside of an Event block, aside from the init global blocks in the current Screen.

Screen1 starts first.

Screen.Initialize is the first event to run in the current Screen.

The components that have Event blocks emit their Events when the user (button click) or Operating system (Clock Timer, File operation complete,...) interacts with them.

None of the component blocks wait for results except TinyDB.

Further info ...

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This is how App Inventor Blocks work:
Understanding an App's Architecture

Also see The model of asynchronous processing in AI2 by Franklyn_A_Turbak

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ok thanks!