Positioning fitted nodes inside fit

Question

I'm trying to draw a nested structure like below. The idea is to make the nodes in the lower box fit perfectly into the drawn outline. How could this be done?

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{fit}
\usetikzlibrary{positioning}
\begin{document} 
\begin{tikzpicture} 
\node (0) {Hello}; 
\node (1) [right=3cm of 0]  { World}; 
\draw (1) -- (0) node [below] {} ;
\node [fit= (1) (0), draw] (4) {}; 
\node  (2) [below= of 0, ] {it's}; 
\node (3) [right=3cm of 2]  {really really here}; 
\draw (2) -- (3) node [below]  {}; 
\node [fit= (2) (3),  below=of 4, draw] (5) {}; 
\draw (4) -- (5) node [right] {} ; 
\node [fit= (4) (5), draw] (6) {}; 
\end{tikzpicture} 
\end{document}

All I can manage to do is this, but here, the structure isn't aligned correctly, because I have to give the position of node 2 before nodes 2 and 3 are fitted together.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{fit}
\usetikzlibrary{positioning}
\begin{document} 
\begin{tikzpicture} 
\node (0) {Hello}; 
\node (1) [right=3cm of 0]  { World}; 
\draw (1) -- (0) node [below] {} ;
\node [fit= (1) (0), draw] (4) {}; 
\node  (2) [below= of 0, ] {it's}; 
\node (3) [right=3cm of 2]  {really really here}; 
\draw (2) -- (3) node [below]  {}; 
\node [fit= (2) (3),   draw] (5) {}; 
\draw (4) -- (5) node [right] {} ; 
\node [fit= (4) (5), draw] (6) {}; 
\end{tikzpicture} 
\end{document}

Answer 1

I assume, that you like something like this:

\documentclass[tikz,margin=3mm]{standalone}
\usetikzlibrary{fit, positioning}

\begin{document}
    \begin{tikzpicture}[node distance = 12mm and 0mm]
\node (0) {Hello};
\node (1) [right=3cm of 0]  {World};
\draw (0) -- (1);
\node (a) [draw, fit=(0) (1)] {};

\node (2) [below right=of 0.west] {it's};
\node (3) [right=3cm of 2]  {really, really here};
\draw (2) -- (3);
\node (b) [draw,fit=(2) (3)] {};

\draw (a) -- (a |- b.north);
\node (c) [draw,fit=(a) (b)] {};
\end{tikzpicture}
\end{document}

Let me note:

• no one your MWE work, missing are essential package and libraries
• used are function not defined in MWE (I remove them)
• from question is not clear, if nodes had to positioned as is shown in above images, or you like to have centered horizontally

Addendum: horizontal centered, well what should be centered? Nodes around texts connected with line? First approximation to this is:

\documentclass[tikz,margin=3mm]{standalone}
\usetikzlibrary{fit, positioning}

\begin{document}
    \begin{tikzpicture}[node distance = 12mm and 15mm]
\node (n1)  {it's};
\node (n2) [right=3cm of n1]  {really, really here};
\draw (n1) -- (n2);
\node (n3) [draw,fit=(n1) (n2)] {};

\node (n4) [above  left=of n3.north]  {Hello};
\node (n5) [above right=of n3.north]  {World};
\draw (n4) -- (n5);
\node (n6) [draw, fit=(n4) (n5)] {};

\draw (n3) -- (n3 |- n6.south);
\node (n7) [draw,fit=(n3) (n6)] {};
\end{tikzpicture}
\end{document}

Answer 2

I think that a simple \rule will simplify the problem because it converts two nodes + fitting node in just one normal node.

\documentclass[tikz,border=2mm]{standalone} 
\usetikzlibrary{positioning, fit}

\begin{document}
\begin{tikzpicture}
\node[draw] (a) {Hello \rule[.5ex]{3cm}{.4pt} World};
\node[draw, below=of a] (b) {it's \rule[.5ex]{3cm}{.4pt} really, really here};
\draw (a)--(b);
\node[draw, fit=(a) (b)]{};
\end{tikzpicture}
\end{document}