Can't get new line at tikz diagram text

Use align=center, or another option such as left or right.

\documentclass[tikz,margin=0.5cm]{standalone}
\begin{document}
\begin{tikzpicture}
    \path (0,1) node[draw,rectangle,rounded corners,fill=green!80]{$P$} (0,-1) node[draw,rectangle,rounded corners,fill=green!80]{$Q$} (2,0) node[draw,rectangle,rounded corners,fill=green!80,align=center]{black \\ box};
\end{tikzpicture}
\end{document}

mostly off-topic (since you already got a solution), however some of suggestions can be useful to you:

• most of you preamble is not related to your problem, please nesxt time try to reduce it to minimum, i.e. provide minimal working example (mwe)
• for similar nodes is sensible to define style, for example rbox/.style = {rectangle, draw, rounded corners, fill=#1, align=center}, which has free parameter for determining fill color
• you can define default nodes color

• give name to nodes (for simple drawings the lines between them)

  \documentclass[a4paper,english,hebrew]{article}
  \usepackage{tikz}
  
  \begin{document}
      \begin{tikzpicture}[
  rbox/.style   = {rectangle, draw, rounded corners, fill=#1, align=center},
  rbox/.default = green!80,
                          ]
  \path   (0, 1) node (p)     [rbox]{$P$}
          (0,-1) node (q)     [rbox]{$Q$}
          (2, 0) node (bb)    [rbox=red!20]{black\\ box};
  \draw[->]   (p) -| (bb);
  \draw[->]   (q) -| (bb);
      \end{tikzpicture}
  \end{document}