What is \null and when do we need to use it?

1. What is \null and when do we need to use it?
   \null is just \hbox{}. When is it needed? Never, actually.

2. Why do \hrulefill and \hfill need starting \null when they follow \\ but not need \null when they follow \par?
   \hrulefill is \leavevmode\leaders\hrule\hfill\kern0pt, so it's not exactly like \hfill, as it adds a null kern after it, but very similar to it. The difference between \par\hrulefill and \\\hrulefill is that after a \par there's a box (the indent box, even with \noindent), but after \\ there's no box on the line, so glue disappears (and leaders, also).

3. Why does \hfill always need ending \null but \hrulefill not need it?
   Because of the \kern0pt at the end of \hrulefill.

4. Why is the first line after \begin{document} not regarded as a paragraph recall that starting \null is needed by \hfill there?
   Could you be more precise about this?

TeX ignores spaces in some situation if there is no material. \null is the same as \hbox{} and it can be used for a material which reserves no space but shows TeX that there is a box which is taken into account for typesetting.

\documentclass{article}
    
\usepackage[a6paper,margin=2cm]{geometry}

    \parindent=0pt
    
    
\begin{document}
    
\hfill xport \hfill\null
    
    

\hrulefill never \hrulefill
    
    

\hrulefill dies \hrulefill
    
    
\newpage

\null
\vfill
    
\hfill The End \hfill\null
    
\vfill
    
\end{document}