The building block view explains your source code in a hierarchical way:

The topmost level (level 1) represents the white box description of the overall system, made up of black box descriptions of the system’s top level building blocks.

Level 2 zooms into the building blocks of Level 1 and is thus made up of the white box descriptions of all building blocks of Level 1 together with the black box descriptions of the building blocks of Level 2.

The following diagram shows a (rather generic) example - starting the hierarchical decomposition from the context view (called level-0 in arc42 terms)

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