Can someone helps me understanding the following code?

I am trying to replicate the code used in this answer

And I am having trouble understanding the following code.

documentclass{article}
usepackage{xcolor}
usepackage[framemethod=tikz]{mdframed}
usepackage{lipsum}

%   Principal MPD frame type
mdfdefinestyle{mpdframe}{
    frametitlebackgroundcolor   =black!15,
    frametitlerule          =true,
        roundcorner     =10pt,
        middlelinewidth     =1pt,
        innermargin     =0.5cm,
        outermargin     =0.5cm,
        innerleftmargin     =0.5cm,
        innerrightmargin        =0.5cm,
        innertopmargin      =topskip,
        innerbottommargin   =topskip,
            }
%   Studies
mdfdefinestyle{studies}{%
        style=mpdframe,
        frametitle={Studies},
    }
newmdenv[style=studies]{studies}

begin{document}

begin{studies}
lipsum[1]
end{studies}
end{document}

Firstly, what does innertopmargin=topskip means
And secondly, why the framemethod has to be tikz instead?
The answerer said that the tikz is used but where?