How to select whole environment in TeXstudio

As benibela from the TeXstudio development group answered:

The cursor has to be placed directly at the border of the parenthesis/environment. I.e. if | marks the cursor, it has to be either there: |\begin{example} or there: \begin{example}| (where it is highlighted)

So, if you have the following code snippet

\begin{itemize}
  \begin{enumerate}
    inner content
  \end{enumerate}
\end{itemize}

and place the cursor like this (notice the yellow highlighting):

you can use Ctrl+Shift+P, I (for Parenthesis/Inner) to get

or Ctrl+Shift+P, O (for Parenthesis/Outer) to get

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Also available is:

• Ctrl+Shift+P, J to jump to the corresponding begin/{ or end/},
• Ctrl+Shift+P, M to find a mismatching brace.