Wolfram Language expressions are nested structures of heads and arguments. They may not appear that way when they are displayed, since many expressions have special display forms to make them more readable.

As an Expression...

You can see an expressions underlying structure using FullForm:

The FullForm expression is equivalent to the typeset version:

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As a Graphical Tree...

Show an expressions structure graphically using TreeForm:

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You can index parts of an expression using the locations of its parts in its FullForm equivalent: