Fancyhdr - Why do changes in headheight influence the footer as well as the headheight in different pagestyles?

Question

I'm a bit confused with the behaviour of fancyhdr.

1. When I change the \headheight for a \fancypagestyle to include a picture (here a tikz placeholder), why is the footer as well as the bottom margin not respected, but also moved (see first page of output)? How can I fix that?
2. Why does the \headheight of the first page with one \fancypagestyle seemingly influence the headheight of the second page with a different \fancypagestyle and different \headheight (see second page of output)?

A MWE and its output follows. Thanks in advance!

\documentclass{article}
\usepackage{fancyhdr}
\usepackage{lipsum}
\usepackage{tikz}
%header first page
\fancypagestyle{firststyle}{
\setlength{\headheight}{130pt}
\fancyhead[L]{}
\fancyhead[C]{\begin{tikzpicture}
    \draw[fill=red] (0,0) rectangle (\textwidth,5);
\end{tikzpicture}}
\fancyhead[R]{}
\fancyfoot[C]{\thepage}
\renewcommand{\headrulewidth}{0pt}
}
%main header
\fancypagestyle{main}{
\setlength{\headheight}{10pt}
\fancyhead[L]{Test Left}
\fancyhead[C]{}
\fancyhead[R]{Test Right}
\fancyfoot[C]{\thepage}}
\pagestyle{main}
%
\begin{document}
\thispagestyle{firststyle}
    \lipsum
\end{document}

The output:

Answer 1

If you keep the geometry set by the class, you must leave a vertical space on the first page, to make place for the expanded heading, and activate the \pagestyle{main} to start from the second page onwards.

\documentclass{article}
\usepackage{fancyhdr}
\usepackage{lipsum}
\usepackage{tikz}
\usepackage{showframe} % added
%header first page
\fancypagestyle{firststyle}{%
%\setlength{\headheight}{130pt} % not needed, will add unnecessary vspace
    \fancyhead[L]{}
    \fancyhead[C]{\begin{tikzpicture}
        \draw[fill=red] (0,0) rectangle (\textwidth,5);
        \end{tikzpicture}}
    \fancyhead[R]{}
    \fancyfoot[C]{\thepage}
    \renewcommand{\headrulewidth}{0pt}
}
%main header
\fancypagestyle{main}{%
    \setlength{\headheight}{20pt} %changed
    \fancyhead[L]{Test Left}
    \fancyhead[C]{}
    \fancyhead[R]{Test Right}
    \fancyfoot[C]{\thepage}}
%\pagestyle{main}
\begin{document}
\vspace*{120pt}
\thispagestyle{firststyle}
1.8.    \lipsum[1-8]
\pagestyle{main}

\lipsum

\end{document}

As an alternative, if you want to control the geometry of all pages and only insert the figure in the first page:

can be done using geometry

\documentclass[12pt]{article}
\usepackage{fancyhdr}
\usepackage{lipsum}
\usepackage{tikz}
\usepackage[top=30pt,bottom=30pt,left=80pt,right=80pt,includeheadfoot, headheight=2ex, headsep=20pt]{geometry}   %added
\usepackage{showframe}  %added
\renewcommand*\ShowFrameColor{\color{blue}}    %added
%main header

\fancypagestyle{main}{%
    \setlength{\headheight}{2ex} % changed
    \fancyhead[L]{Test Left}
    \fancyhead[C]{}
    \fancyhead[R]{Test Right}
    \fancyfoot[C]{\thepage}}
\begin{document}
\vspace*{-45pt} 
\noindent\begin{tikzpicture}
\draw[fill=red] (0,0) rectangle (\textwidth,5);
\end{tikzpicture}
1.8.    \lipsum[1-8]
\pagestyle{main}

\lipsum

\end{document}