Footcite all Authors

Question

I need the footcite to display all authors like this. However I only got so far that it shows only the last name of the first author. In this case "Horschitz". Here my code.

\documentclass[12pt,a4paper]{article}
\usepackage[german]{babel}
\usepackage[backend=biber,style=authoryear,giveninits=true,sorting=nyt,]{biblatex}
\usepackage{filecontents}
\AtBeginBibliography{%
  \renewcommand*{\finalnamedelim}{%
    \ifnumgreater{\value{liststop}}{2}{\finalandcomma}{}%
    \addspace&\space}%
}
\let\origparencite\parencite
\renewrobustcmd{\parencite}{%
  \AtNextCite{%
  \renewcommand*{\finalnamedelim}{%
    \ifnumgreater{\value{liststop}}{2}{\finalandcomma}{}%
    \addspace&\space}%
  }%
  \origparencite%
}
\DeclareNameAlias{sortname}{family-given}
\begin{filecontents}{myreferences.bib}
@book{bilstr,
    address = {Stuttgart},
    author = {Horschitz, Harald and Gro{\ss}, Walter and Fanck, Bernfried and Guschl, Harald and Kirschbaum, J{"u}rgen and Schustek, Heribert},
    date-added = {2021-03-27 23:38:45 +0100},
    shorttitle ={Bilanzsteuerrecht},
    date-modified = {2021-03-27 23:41:36 +0100},
    edition = {14},
    publisher = {},
    series = {Finanz und Steuern},
    title = {Bilanzsteuerrecht und Buchf{"u}hrung},
    volume = {1},
    year = {2016}}
\end{filecontents}
\addbibresource{myreferences.bib}
\begin{document}
This is an example text \footcite[Vgl.][S.33]{bilstr}
\printbibliography
\end{document}

Answer 1

Set maxnames to a high value like 999 to display all name. Set the labelname format to sortname to display the given name initial as well.

I also added some code to add slashes between names in the citation. I explicitly only changed this for \footcite and not for the bibliography and \parencite, because your MWE has different code for those. (I slightly modernised that code, but left the function intact.)

\documentclass[12pt,a4paper]{article}
\usepackage[ngerman]{babel}
\usepackage[
  backend=biber,
  style=authoryear,
  sorting=nyt,
  maxnames=999,
  giveninits=true,
]{biblatex}
\DeclareDelimFormat[bib,parencite]{finalnamedelim}{%
  \ifnumgreater{\value{liststop}}{2}{\finalandcomma}{}%
  \addspace&\space}
\DeclareDelimFormat[footcite]{multinamedelim}{\addslash}
\DeclareDelimAlias[footcite]{finalnamedelim}{multinamedelim}
\DeclareNameAlias{sortname}{family-given}
\DeclareNameAlias{labelname}{sortname}
\begin{filecontents}{\jobname.bib}
@book{bilstr,
  address    = {Stuttgart},
  author     = {Horschitz, Harald and Groß, Walter and Fanck, Bernfried
                and Guschl, Harald and Kirschbaum, Jürgen and Schustek, Heribert},
  shorttitle = {Bilanzsteuerrecht},
  edition    = {14},
  series     = {Finanz und Steuern},
  title      = {Bilanzsteuerrecht und Buchführung},
  volume     = {1},
  year       = {2016},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
This is an example text \footcite[Vgl.][33]{bilstr}
\printbibliography
\end{document}

If you prefer a more consistent output, replace

\DeclareDelimFormat[bib,parencite]{finalnamedelim}{%
  \ifnumgreater{\value{liststop}}{2}{\finalandcomma}{}%
  \addspace\&\space}
\DeclareDelimFormat[footcite]{multinamedelim}{\addslash}
\DeclareDelimAlias[footcite]{finalnamedelim}{multinamedelim}

with

\DeclareDelimFormat{multinamedelim}{\addslash}
\DeclareDelimAlias{finalnamedelim}{multinamedelim}

I'd also like to point you to my answer to one of your earlier questions. In particular I recommend you drop the "S." in the postnote and let biblatex add it for you. You can also use ü and ß directly instead of {\"u} and {\ss}.