I think that a good solution for How to put a line above the lower part of tcolorbox and How to make lower part of tcolorbox as tall as the title could be to replace the lower
text with a tcbsubtitle
. But as you can see in following example, this introduces a vertical empty space between the subtitle and lower border. Do you know where is this space defined and how to avoid it?
\documentclass{article}
\usepackage[many]{tcolorbox}
\usepackage{lipsum}
% --
\begin{document}
\begin{tcolorbox}[
title={Title},
center title,
toptitle=1mm,
bottomtitle=1mm,
sharp corners,%
colback=white,
colframe=black!75,
]
\lipsum[1]
\tcbsubtitle[halign=center]{Text}
\end{tcolorbox}
\end{document}