How to send edges to back in tikz trees

Alternative solutions with use of the forest package. Maybe someone will like it.

Case A: nodes have one line text (as shown in question), consequently image in portrait oriented page is a wee bit wider than page width. Therefore this solution is appropriate for landscape page:

documentclass[margin=3.14159mm]{standalone}
usepackage{forest}
usetikzlibrary{arrows.meta}
tikzset{lbl/.style = {inner sep=2pt, font=footnotesize, midway, above}}

begin{document}
    begin{forest}
for tree = {
% node style
    rounded corners,
    draw,
    minimum size = 1em,
            font = small,
% tree style
    edge = {-{Straight Barb[angle=60:2pt 3]}, semithick},
    edge path'={(!u.parent anchor) -- (.child anchor)},
    l sep=12mm,
    s sep=1mm
           }
% tree body code
[root,
    [ , circle, edge label={node[lbl] {Hard-ware}},
        %%%%    settings for the right branch
        for tree={grow=east, anchor=west, child anchor=west},
        before computing xy={l=-3,s=+22mm},
        %%%%
        [Industrial control systems devices cite{myers2018anomaly}]
        [Ubiquitous computing devices cite{yousfi2019discovering}]
        [Geotags cite{diamantini2017discovering}]
        [Eye tracking devices cite{ioannou2020mining}]
        [Ventricular assist devices cite{movahedi2019sequential}]
        [WiFi-base indoor positioning systems cite{liu2014proactive, hwang2017process}]
    ]
    [ , circle, edge label={node[lbl] {Soft-ware}},
        %%%%    settings for the left branch
        for tree={grow=west, anchor=east, child anchor=east},
        before computing xy={l=-3,s=-22mm},
        %%%%
        [Relational databases cite{syamsiyah2020process} cite{andrews2020quality}]
        [Electronic data interchange messages cite{engel2016analyzing} cite{krathu2014framework}]
        [Natural language texts cite{metsker2018identification} cite{kabicher2011human}]
        [Electronic medical records cite{metsker2017pattern} cite{chen2019mining}]
        [Educational platforms cite{cerezo2020process}]
        [Librarian systems cite{kouzari2018process}]
        [Software development projects cite{samalikova2011toward}]
        [Different information systems cite{myers2018anomaly} cite{schuh2020data}]
    ]
]
end{forest}
end{document}

Case B: nodes have prescribed text width, consequently text in nodes is broken into more lines. With this the image can be adjust that is narrower than text width and therefore can be presented on portrait oriented page.

documentclass[margin=3mm]{standalone}
usepackage{forest}
usetikzlibrary{arrows.meta}
tikzset{lbl/.style = {inner sep=2pt, font=footnotesize, midway, above}}

begin{document}
    begin{forest}
for tree = {
% node style
    rounded corners,
    draw,
    minimum size = 1em,
% prescribed nodes width and font size
        if level = 1{}{if level = 0{}{text width = 10em, /tikz/align=flush center}}{},
            font = linespread{0.84}selectfont,
% tree style
    edge = {-{Straight Barb[angle=60:2pt 3]}, semithick},
    edge path'={(!u.parent anchor) -- (.child anchor)},
    l sep=12mm,
    s sep=1mm
           }
% tree body code:
%  code for tree is the same as is in the above example
end{forest}
end{document}